Thanks for all the help, I'm making some progress!
Budget is fairly flexible - I'm not someone that chops and changes often and I'm a one MTB person. The Boardman was my first after a thirteen year break, I've had it for about 18 months, bought it expecting to just being doing XC stuff round the South Downs but have found myself mostly doing more aggressive singletrack riding in the woods and visiting trail centres, thus it's been modified to suit but there's only so far you can go. Having broken my ankle I'm concerned that I might be tempting fate if I continue to try to go faster and faster down steep rocky places on a bike that isn't really built for it...
Anyway, I want to get this bike right and leave it be (I've done the same thing with bass guitars - starter bass for a few years (but I was starting from scratch), then a rather nice secondhand production bass that I played for about ten years, then an uber-niche custom-made oddity that I've had for the last few years) so if for instance the Sektors are too compromised I'm willing to spend more - I'd rather pay more once than have to upgrade.
I'll go with the Hope headset. (The Boardman headset is an FSA semi-integrated one so I don't believe it'll fit).
I'll take this rear hub apart and see what condition it's in. If it's all good I'll replace the cartridge bearings and leave it be.
Lower profile pedals would be nice but I wouldn't want bigger ones - I'll run these V12s until they die, they stick to my AM40s very well - and then look for something thinner. I'd like wider bars if it wasn't for the trees round here - go past 700mm and you're forever getting stuck! As it is I don't use lock-on grips and I ride with my hands as far out on the bars as possible, sliding in a bit when it gets too tight to fit through without bruised knuckles.
So, more questions: I had totally overlooked the front mech issue - I thought all I'd needed was a shim to fit the skinny seat tube but as the cable routing is from above rather than below it's a different pull. Damn! So if I understand correctly I need a top pull 28.6mm front mech - any reason not to go for an X9 one of these? As I ride double+bash and have no intention of ever returning to a triple, can I get a double specific front mech?
Wheels: I have almost no idea where to start: Hope Pro 2 front hub makes sense. I guess I'll need a set of shorter spokes too. Will investigate the costs of that vs a fully built wheel. It does look though that once I've bought the parts and the tools it might cost more than buying a whole wheel, so the economics of that depend on how many wheels I'm planning to build in the future. With a new wheelset it will have a new rim - my current rims are skinny XC ones (some Ritchey WCS thing) which are very light and I've never had a problem with them but then again with my skill levels there's only so hard you can hit things on a XC HT and stay on.
I like using quite fat tyres (Bontrager XR4 2.2 - bit wider than a High Roller 2.35 and quite a bit taller) so would wider rims be wise? I guess this is where I hit the "light, strong, cheap, pick any two" dilemma. I'd like light-ish, strong-ish and cheap-ish please! I run ghetto tubeless so I think any rim will work with that - any benefit to UST/TR rims?
Right, the forks dilemma:
Sektor 100-140 coil u-turn - cheap, presumably nice and active with the coil spring, middling weight, simple, low maintenance, too bobby when standing and pedalling?
TALAS 32 100-140 - expensive, tweakable, light, better damping, high maintenance
Magura Thor - don't know much about them, advice please!
15mm or 20mm bolt-thru? Presumably all these heavier-duty forks have much more tyre clearance than the Rebas I currently have - I like to run big tyres all year round so the more mud clearance the better! Very hard to find clearance specs on the manufacturers' sites.
Sorry that got so long - my esteemed thanks if you've bothered to read this and can help me out!