Let's offroad / mountain bike / mtb / ride dirt

Yes - suspension makes for a more versatile ride.

29ers seem to have found their niche in XC racing. There are some 29er trail bikes but they are in the minority.

That said try it - you might like it.

I know what I'm doing with bikes generally but MTBs are totally new to me - so I'd have no idea looking at a used suspension fork or disk brake whether it was buggered or not. And generally, after only ever having bought 2nd hand or building up a frame, I'd rather buy a new complete bike this time.

Yes

And in terms of versatility, it needs to be fine for general off road, towpath, the trails near Bristol (generally not especially gnarly I think) and essentially be as much of an all rounder as possible. Would 650b really be that much better than 29er for that? The difference wouldn't be so huge would it?

650 / 26" bikes tend to have more travel

All things equal they are stiffer and lighter.

That said a well thought out 29er would be fine. nice and stable, plenty of grip.

There's some gnarly stuff around Bristol - ask @LHL

My mate has a bizango and for the money it's decent.....but anyway, 29ers are faster. Unless you are really going to be slinging it into berms and doing jumpy stuff all the time, or putting into a car and going to a 'trail centre' (a cuntish label, if ever there was one).
I won't ride a 26" anymore, as 29" is faster......it has transformed my bike riding over the last 5years. 27.5" is nothing more than a compromise, bought about by the marketing bollox written in magazines.......who also are responsible for "trail centres".
So, to end my angry post, my summary in a nutshell..
26" if you are young and want to do jumpy/hurty stuff
29" for the grown ups.

That ought to get the flames going......I'm fucking grouchy today

I like my 29er when it's got a race number on it

I like my 29er when I'm removing the race numbers from it.

Wow, I've just read that back, and it really shows my pent up aggression from not being able to ride my bike for four days, and having my mother in law round my house, the provocative fucker......

I still have to wait another 23.5 hours before its bike o'clock too!

Breathe in.....

And out....

Inhale through the nose....

And out through your mouth.....

Your ranting needs work, that was barely worth the breath.

I am very fortunate to get along with my mother-in-law.

My previous one, on occasion, had the ability to render me unable to form coherent sentences.

Hah!

I'm nice by nature though.....

It's not her fault.....she's an only child.

Here's a reason to visit South Wales, and to use 650b:

https://www.youtube.com/watch?v=uLfOXk72FkY

Where do you usually ride?

^^ Looks a nice trail though not wheel size specific or a race stage.

Which trail?

The trail shall remain unnamed but is not too far away from where I live. @Howard has been there.
It's steep, rooty, has some radical offcamber in places, and some very tight switchbacks.
Wheel size is more a matter of preference but for me 650b is better for negotiating these kinds of trails.

Aha - 'cheeky'?

Edited - didn't mean to sound such a knob.

This is LFGSS. Being a knob is de rigeur.

Though I think you are over estimating the knob content of your previous reply.

First ride around Bedgebury today. Was pleasantly surprised, not a massive quantity of trails but what's there is of a good quality and looks like it stands up to a fair amount of usage, given how busy it was today. What became apparent was that my summer micro-knobbed tyres, even on the rear, whilst fun to drift around corners, are a bit sketchy and need replacing.

Would the Specialized Storm Control (2") on the rear, and Purgatory (2.3") on the front be a good combination for the remainder of winter and spring?

The Storm might even be overkill depending on how muddy your riding is. The (26") Purgatory I had came up more like 2.1, so bear that in mind, but it was a decent tyre

I'm running a Purgatory on the front now, it's definitely not 2.3" and might just about squeeze in my seat stays but I've found they clog up quite badly with the clay found around Epping. Okay on the front where I've got loads of clearance but may be an issue on the rear.

I suppose the other option for the rear (from Specialized) is the Ground Control, which is more of an all rounder. I tend to ride the cross bike if I'm expecting loads of mud, so that would be more sensible.

the Butcher has a more open tread, that on the front with a Purgatory on the rear is a common combo but I can't think of any tyres that won't clog in Epping slop beside full-on mud tyres