Any question answered...

stack and reach is the only sensible way to compare

Lo pro sizing

Pick your handlebar first and work backwards from there. Bullhorns vary so much in stack and reach that you have to pick one before you can tell your frame builder where to put the stem. Once you know where the stem goes, you can choose a compatible front wheel size.

The bit as the top is the lock nut. I can't use the rest of the threaded headset as it's all bearings and cups. Should I get another lock nut to tighten it to?

Straight up foolishness from me.

Should I get another lock nut to tighten it to?

You need another nut, but it will not be a headset lock nut as they overlap the tube OD at the top.

What you should have done from the start is use the complete threaded headset with spacers between the upper cup and the lock nut to take up the surplus steerer. It's #sheldon_approved :-)

But there is no threading where the threaded top race sits...hence why I used a threadless headset for that bit.

I just need a smart way to stop the lock nut at the top unthreading itself. I've been googling 1 1/8" headset spares but they all have flanges. I just want a little lock nut to tighten it to but without the flange like the one on the adjusting cup below:

If my crank arms (which were fine) are now (following an accident) hitting the frame, is it more likely on balance that the steel frame has bent, or that the crank arm has bent?

Presumably the cranks are aluminium? If so, they are more likely to break than bend, so it's probably the frame.

But there is no threading where the threaded top race sits...

Good point well made :-)

I just want a little lock nut

It wouldn't be a difficult thing to make. Simplest method would probably be to mill or face off a standard lock nut to remove the rim which is stopping it from running down past the top of the tube.

is it more likely on balance that the steel frame has bent, or that the crank arm has bent?

If it's a square taper, it's at least as likely that the axle has bent as either of your propositions.

Have you been to HK before?
if not, ensure you leave the airport with a bottle of water.
Airport Express Train into Central.
As a Man of cultured tastes you probably want to have a look at Richard Rogers HSBC building. (Free escalators from gound level ino the atrium/reception area).
Walk to Ferry across to Kowloon, (ferry terminal has been re-located, but the old ferries till plough across what is left of the (repeatedly infilled) Victoria Harbour.
In Kowloon note all the guys offering you hand made suits in 24/48/72 hours.
If rested, wander over to Chunking Mansions:
https://en.wikipedia.org/wiki/Chungking_Mansions
Get the MTR metro to Mong Kok.
https://www.google.co.uk/maps/place/Cordis,+Hong+Kong/@22.3180306,114.1691173,17z/data=!4m5!3m4!1s0x340400c784bc08cf:0xcad7219666b937d7!8m2!3d22.318239!4d114.168141
Note Langham Place tower block/shopping centre with international coffee franchises in the public spaces. Other side of road leads to Tung Choi Street for the 'Ladies Market',
(really just an all day street market), for all your counterfeit needs. The cafes 'behind' the stalls are fine to eat in, just don't expect English to be spoken everwhere.
The foul/sewage smell sometimes signals edible street food, (I think it is boiled, aged tripe that gives the distinctive smell, but all the rest is fine). As are the places offering fresh fruit juices while you wait.

Walking takes much longer than in London, partly due to the sheer volume of people, and their inability to walk in straight lines.

Wander around the street grid and return to Mong Kok MTR back to Central and the Airport Express. You will have to be back at the Airport before the nightly, 20.00, lightshow
http://www.discoverhongkong.com/uk/see-do/highlight-attractions/harbour-view/a-symphony-of-lights.jsp

Haha

Yeah I guess so. I don't have access to that equipment though.

Are these really not made already?

Rotor 3Df BSA bearings... Do I need the whole cups or just the sealed bearings? Where can I get them? Links please.

Thanks peeps!

I don't have access to that equipment though.

Everybody has access to that equipment, it's called a job shop :-) There's minimal set up and no programming, so any kind of manual lathe or mill will do the job.

I want a big block of metal with a concave cut out which corresponds to the outside diameter of a headtube.

I'm sure it or something close enough will exist already as a tool for another job but no idea what to look for or where, anyone?

Can you not use a standard V block, the usual method for supporting round objects in machine shops.

I want to use it to press metal into the correct curvature to for a headbadge so the need the curve really.

I've now found smaller versions, up to 3/4" diameter (think I need around 2" dia) going by the name of swage blocks or dapping blocks so hopefully I'll find what I'm after sooner or later.

In that case you can probably manage with a couple of pieces of thick wall pipe, one whose id is the same as the head tube od. Cut this in half length wise and press into this using another piece of pipe of the requisite OD. Unless you are using very thick material for the badge then you may need something very robust.

I'm using some sort of pipe at the moment (it was left in the garage when I moved in) with old sockets to press into it but finding it surprisingly soft considering how much effort it too to cut it up. Was hoping for a more long term solution but might just have to visit b&q/homebase and see if they can offer anything that'd be an improvement.

Is what I really need but £250!

A leather sandbag might do the job...

Have you been to HK before?

Nope, thanks for the advice

You'd get 3 or 4 hours wandering around, train is about half an hour to Central. I'd do it if you haven't been to HK before, not sure if I'd bother otherwise. It's a nice enough airport to hang around at, particularly if you can use the lounge.

I can use the lounge, but I'd prefer to nip into HK never having been there before.

In that case I'd have a quick blast around the centre. The Airport Train to Hong Kong Station takes about half an hour and is pretty reliable

Forgot to say, on the return journey,
no need, of course to use the Ferry,
the MTR can take you straight to Central / Hong Kong interchange

http://www.mtr.com.hk/en/customer/services/system_map.html

or you could go 'North' and pick up the Airport Express at Lai King.
http://www.mtr.com.hk/en/customer/services/system_map.html
(MTR is great, English & Chinese announcements,
and illuminated lights showing you the direction of travel on maps above every door,
and which side the doors will open at the next station).