after spending a few hours scratching my head, shining a torch in the case and muttering under my breath, I eventually split the cases again.
I then stumbled upon the correct technique of getting the gearbox back in with ease.
Although, I had the circlips and washers in the correct order, I didn't take into account that one of them should go on the outside of the gearbox casing, so that when the shaft pokes through, its a washer, gear, washer, circlip combination.
Doh -
Anyway, all (nearly) back together now to finish off tomorrow