As above I pulled away in first, was moving at about 5mph when I applied a spot more throttle to be greeted by a clunk and it coming out of gear (the clunk seemed to be from the centre of the car at the front as it were, and my initial thoughts were that the clutch cable had gone). Attempting to put it back in gear however showed the clutch to be working fine, however it once again jumped out of gear on the application of the loud pedal.

It then started to behave reasonably well after the brief disobedience if it felt a little down on power, however gearchange feels decidedly different, much less smooth and somewhat notchy in relation to its previously smooth state.

The gearbox mount stood up to a prybar testing, as did the engine mounts, the chassis hasn't bent, and the engine mounts seem ok. Any ideas where to look next?

Many thanks.

It could be clutch adjustment. Best way to test, select first (u need feet and hands) then get a suitable amount of revs, quite a few, and wedge your foot on the break. Also handbrake off. I suggest throllte with your hand, foot on brake and other foot on clutch. After building a few revs dump clutch. But make sure you have your foot on brake. If the car stools straight away clutch = good. If not straoght away clutch is bad

it could be needing adjustment, as wehn you have your foot on clutch it could not technically be dis engaging completly, Or the other way. When you have foot on clutch could be slightly un engaged. Causing slip which COULD cause it to jump out or something if that makes sense

im sure the experts will say that its not clutch and tell me im a muppet but thought id say it anyway !

Deffo do the clutch test. Also look for a small amount of play in clutch cable at pedal. By pinging. Also the first 10cm i belive on the clutch should be easy to press.. with out much force

A couple of ideas to check.
Difficulty going in to gear can be as simple as the gear lever on the top of the 'box working loose, check that the knurled ring on the gear stick is still level with the end of the slide tube when in neutral. It's possible that if it isn't going into gear properly it'll jump out easily. This would affect all gears

The gearbox should keep it's self in gear with 'detent balls', a ball bearing held against a cut out in the selector rods buy a spring, there's one for each rod i.e. one covers 1st&reverse, one 2nd&3rd, and one for 4th. They not only keep the 'box in gear but also create the feel of it going into gear. It's unusual but possible that one of the springs has broken, if it's the one that covers 1st&reverse then both these gears will be affected but all others should be fine.

The clutch is fine.

A play with the gear lever made no appreciable difference, so I took it for a spin to get it warmed up and found that if one moves the gear lever gently and rather slowly it semms more or less fine (if again perhaps slightly less smooth than it used to be).

I then removed the drain plug to look for lumps on the magnet, and found nothing other than the usual spikey haircut, replaced the gearbox oil (it was rather black and mucky), and went for a spin.

The lack of power has now been attributed to accelerator cable stretch, a spot of adjustment had the thing going at 75mph on the test run along the dual carridgeway, whereas before it said no at 60mph.

As at present it seems to be ok I shall put off taking the lid off the thing until I return from holiday (leaving tomorrow morning for a 200 mile trundle to pembrokeshire, and then back a week or so later), unless anybody thinks the thing is liable to self destruct over the next 500 miles or so. Would it be worth slinging a spare gearbox in the boot?

On an odd note the gearbox took one whole litre of oil before starting to overflow rather than the prescribed 900ml. Perhaps I didn't have the car entirely level in retrospect, would this be likely to end the world? (I have always attempted to steer clear of gearboxes).

Many thanks again to all for their thoughts and advice.

Begining gear box problems should not be ignored. It may take time maybe not before worth is to come. Yes bring a spare box.
I don't have enough info (yet) but it could be the lock balls being worn and/or the braking clip. Good luck with the trip

Braking clip jammed.

Many thanks, I shall add a spare box to the boot (I would have a rummage through the installed one now but I think the girlfriend would kill me).

Are there any further tests I can perform to illicit more information without removing the lid?

It can be hard to diagnose from a computer. If you are not completely familiar with a gear box maybe take the car to someone for a test run.

Splendid.

I've dug a spare out of the shed and put it in the boot. It feels wonderfully smooth changing with the stick ontop of the box, and if I remember rightly was said to have come out of low milage dyane when I got it. Should suffice in emergency though I guess.

Would there be a particular book on 2CV gearboxes that it might be worth investing in?

You need the ''original'' A model rep. manual to work on the gear box. Best left to the specialist.
Something to look at here
http://www.2cvstuff.com/Files/2CV/108161/8161CH05.PDF

Worn bearing(s) allowing the main cluster to move and so disengage gear( the selectors stay put but the cluster moves and so jumps out of gear

the input shaft can fail in that the nylon race breaks and all the balls dead off to one area this usually has the box howling and juddering

Top rear bearing is the one that goes or the nut becomes loose and the loss of power is the shaft trying to exit the rear of the box

Sean