Got the van back and it's driving like new.
Lovely positive gearshift, and definately fully into 3rd gear now each time. No crunching at any revs and feels much better in the other gear changes too - probably due to the new oil in the box.
The fix was a new selector fork - I'm told its actually quite an easy thing to replace once the box is off the van, so the main cost is the labour to remove and refit the box.
The specialist who did the work reckons its very common on Transits - the selector fork loses its case-hardening and then wears away in no time.
Another good thing, is that the rest of the box has been given a clean bill of health too - no other wear to worry about.
I bought the van from Stanton Commercials in Coventry, and I have to say I give them full marks for their aftersales service - they sorted this issue without hesitation, I'm very impressed with them.
I didn't ask how much it would have cost though - seemed a bit impolite after all their efforts!