Have filled in the grid with a couple of guesses for two unclued (15a and 39a) but not confident. Is the theme that the second letter of unclued are either x or y?
No - You need the latest Chambers because Word Wizard doesn't help (Universal Crossword Solver does). The unclued are all the final entries under specific letters.
Thanks for your help Novalis. I'm still at sea here - could you clarify by what you mean by final entries under specific letters. I don't know what the them is which obviously doesn't help!
There is no actual 'theme'. The unclued are all the last entries in Chambers under 8 different letters. I assume you have 3 down. That is the last entry under B. 15 across id the only unclued with an unchecked first letter and that is F.