Ask around for the best one in the UK .... I have no idea what you've got over
Maybe better asking around in France, since that's where he is...
Unleaded fuel goes 'stale' quite quickly, a couple of months may be enough for it to have gone off a bit.
If it's really rough, I'd be tempted to drain the tank and refill it with fresh fuel and an additive as Patrick recommends. Good advice about draining the carbs too, as any water will have collected there.
If it's not too bad, you may get away with diluting the old fuel with new as you top up as normal. I'd still drain the carbs though.
Also, you could remove the air filter and spray an aerosol of carb cleaner into the intakes to help remove any gunk and varnish that may have accumulated in there. You should be able to get this in any car parts store.
Who told you it was a 15 hour job to get the carbs out..?
Was it the dealer by any chance...?!
Beware the dark, mechanical bird.