Did around 40 miles on the bike this evening. Very happy with the gearbox, feels great. I think I've got the selector lever one spline too high compared to before but easily sorted.
I did notice the clutch slipping, quite badly at times so I really need to tighten that up. Is there any rule of thumb for how much to do it? What I should have done was measure how much thread was showing beyond the locknuts on the 6 springs before I took it apart as that would get me in the right ball park as it was fine before... hindsight is a wonderful thing.
I suppose I can kick it over against compression and feel for the bite point moving as I tighten the springs up.

So I found the time this afternoon to get the new bushes in the gears and pinion sleeve, ream them out and sort out the end float on the shafts. Very happy with the selection now, although in neutral the 2nd gear dogs were closer together than I would like. This was assembled just with the gear control shafts and detent in place - no ratchet or change pedal mechanism fitted yet. I took it apart again and measured the height of the dog teeth from the fork grooves - a selective fit with the 'shortest' dog teeth adjacent to 2nd gear seemed to gain me around 20 odd thou extra clearance which made things a lot more comfortable, yet still allowed full engagement in all gears.
Tomorrow I'll do a final assembly and hopefully get it refitted into the bike. The primary chaincase is being painted whilst it is off also so it'll be a week or so before that's ready to go back on (especially as I am away this coming week).

Hopefully it'll stop raining by the time it's back together!

I stripped the gearbox this evening. Generally good condition, with the usual wear on the pinion bushes (I think I will be rebushing them, and the pinion sleeve too). All three shafts mic up correctly and there's no obvious damage to them.
The 3rd/4th selector dog is worn/galled in the groove where the fork runs, and the fork is worn too (around 15-20 thou of wear on the fork at the worst point) The peg in the track of the selector fork is good.
The end float of the gear control shaft I measured at 60 thou with a DTI once I had removed the outer cover.
The end float on the layshaft was excessive too, but I have not measured it (the mention of drilling a hole in the outer bush to measure end float is a good idea, I may well do that)

Someone has definitely been in there before me though. The oil flinger washer which should go between the 1st speed mainshaft fixed pinion and the spacer was found on the outside of the mainshaft bearing, between it and the kickstart ratchet mechanism. It has heavily dished and rubbed against the circlip/snap ring which holds the bearing in the inner cover. As a result the mainshaft has been sitting too far towards the right (by the thickness of said flinger washer) and as such the 3rd speed pinion on the mainshaft was even further from the selector dog. Coupled with the wear on the fork and the excess endfloat on the control shaft it meant that the selector dogs could slide out of mesh as found on my initial peek through the inspection window!

I'm currently waiting on some spares to be delivered, and fingers crossed the selector forks and dogs in the spare box will be a little better than mine. Even if they are not, I think correctly shimming the control shaft, and correctly assembling the mainshaft will solve the problems I have been having.

Hi Dave

Thanks for the reply, I did think the tube would move sideways but it didn't seem to want to when I tried... at least you've confirmed that it should so I'll give it another go and knock the bearings out. Yes the bearings will be getting replaced, fear not!

Thanks
Calum

Hermes are hopeless. I hope I never have to deal with them ever again!

Different sized sprockets will also affect the reading, if the drive is taken from the gearbox (obviously different primary drive gearing wouldn't affect the speedo)
As said different tyres and pressures will affect it also, but I have to say 3 miles over such a short distance is a big discrepancy - near enough 10%

Glad it all worked out.

My suggestion was from experience, I recently had the same problem on my partner's Ariel NH. The lifter was slightly binding and once removed revealed the tappet was set to an alarmingly large clearance! With the lifter fitted it felt okay... I reset the tappets and refit the lifter. Took a few goes to get the right amount of clearance before it touched the rocker arm - a much better design on the B31/B33

If you're getting 6v at the terminal in the bulb holder with the switch pulled then the wiring up to there should check out... SHOULD.
Multimeters will show a voltage reading when the current is so so very low, so sometimes a dry/broken joint might still show as transmitting power but as soon as you actually load the circuit up (with a bulb or whatever) then sometimes the joint then 'fails' and there's 0v. In many cases a test light is mich better for fault finding (easily made with a bulb holder, some wire and a pair of crocodile clips or similar)

On the other hand I think your issue is probably an earth issue, or the bulb. You said the multimeter was connected to the battery earth. Try the meter across the bulb contact and the base of the lamp unit, or check for continuity between lamp base and battery earth.