I've emailed Day and Whites, but realistically won't expect a reply until next week. I'll be feeling a bit stupid if this isn't a VX gearbox...
I also contacted the seller of the gearbox (who never actually fitted it into his car due to circumstances, just stored it in his garage) and this is what he said:
As I said I bought it off of a club member, and saw the car he was breaking & took the gearbox from and it was a Volumex.
I've no reason to doubt the seller, he seemed a very pleasant and genuine chap, but similarly Eric knows his way round Volumex engines and gearboxes better than anyone I know. So we'll have to wait and see I guess.