Any question answered...

I thought the 68/73 switchover occurred in the 90s

There was no "switch-over", both widths have always existed side by side since 73mm first arrived. The question is moot, since Shimano HT2 MTB cranksets will work with either, they just use a 2.5mm spacer each side between the cups and the shell for 68mm and omit them for 73mm.

Hypothetically speaking, if someone were to go to the garage for a newspaper at, say, 1030pm and find that all the papers had been tied up for collection, and that person took a newspaper, would the attendant get in trouble or would it just be a case of BP losing a few pence?

It’s theft

They're really going for the jugular on Prince Andrew in today's Times

I know you can use spacers. But some people have a preference for road cranks because of chainrings, aesthetics, weight, Q-factor, whatever. "you might be able to use road cranks" was the nub of my comment

"you might be able to use road cranks"

That's going to need some careful measuring to check that the narrower chainline and Q don't interfere with the chainstays

https://www.sigmasports.com/item/Wolf-Tooth-Components/94-BCD-5-Bolt-Chainring/IC54

Alternatively I could ball out on this, which is pricey, but probably not much more than buying new cranks. Though I'll only have an 8 speed cassette in the back, so it feels a bit obscene to spend more than I've spent on my derailer, shifter, cassette and cranks combined on a blingy chainring.

I will measure the BB shell to get some more solid data.

There are loads of “IXF” Chinese hollowtech II style cranks on eBay for ludicrously cheap that take standard MTB chainrings, and quite often come with BBs. I have one on my retro Cannondale and it works fine and doesn’t look out of place.

I need to pull off a crank from a bb where the crank extractor thread is toast. I'm thinking of getting some kind of cheap bearing puller from ebay but wondering if people have tried and had success with any particular model.

I had a bolt sheared off in my mudguard hole on the fork (have randomly forgotten what they are called). I finally (after nearly a year) managed to get the time and tools to drill it through. The original hole is obvs M4, i've drilled through with a 3mm drill.

Question is, should i try and get someone to tap the 'new' hole at either 3mm or 4mm, or am i just best off using a 3mm bolt with a nut on the inside of the fork to secure it? (or drill it through with 4mm and use 4mm bolt and nut)

I’ve tried this and it was made of such flimsy metal all of the force went into destroying itself rather than getting the crank off. Eventually the top casting sheered in half.

The crank is still attached.

My conclusion was you won’t get it off without heat. Find an LBS that has a blow lamp and a GBFO hammer.

i've normally just taken the bolt out and ridden around for a bit, it'll just drop off.

Yeah this. If you don’t want to use the crank arm again because it will die in the process.

Hmm yes it does makes sense they wouldn't be strong enough as a typical bearing wouldn't need anywhere near as much force to unseat as a crank arm.

I am trying to get it off non-destructively. Otherwise i'll just leave it :/

Can you take the BB out? if you're able to take the crank arm with bb attached, you could pop the crank in a vice and then tap out the spindle.

Yes I can. That's an idea, I'll have a look see if I have a suitable punch hanging around.

I made an angled wedge to fit between the frame and the crank with a slot for the axle, this work\ed for me.

Is there a rule-of-thumb on the amount of rear (or indeed front) derailleur 'slop' at which it should be considered dead?

Have various old ones in storage, no memory of whether they had shift issues, and don't want to fit them on a bike before selling/binning.

Sykes-Pickavant sell a very similar looking automotive tool - a ball joint splitter.
I'm pretty sure I have one - @Tychom you are welcome to borrow it - I'm in SE London.

The original hole is obvs M4

Rly?
They are usually M5 (very occasionally M6). If you drill through 4mm, you'll probably be able to peel out the remnants of an M5 screw.

wondering if people have tried and had success with any particular model.

This pattern has always worked well for me on cranks, also does many headset crown races.
https://www.lfgss.com/comments/8244998/incontext/

So, is there any consensus about which cycling organisation to join for 3rd party insurance, rather than British Cycling? I'd go for LCC, any better suggestions? Both with regards to value for money, and usefulness of the organisation over all - e.g. I prefer a focus on extending cycling infrastructure and the like, rather than advertising Team GB.

Just the sort of response I was looking for. Will drill 4mm and then take it from there. I did wonder why the remnants weren't so forthcoming with the 3mm hole drilled.

Thanks

Spray some WD40 or GT85 before you drill and leave it overnight.

The broken bolt might just undo itself when you start drilling and pop out of the other end. Saves you from removing any remnants or re-tapping the threads

Thanks, will do. I think the reason the bolt sheared in the first place is because the thread wasn't tapped/faced when I got the frame as new