Thanks Ged - would you and others mind if I just tested my understanding? I deliberately have not so far looked elsewhere on this thread; my intention is to go in later on remaining clues. So apologies.
My reading of the (opaque) instructions was that A need to be added to one of the clues in 7 across, B to one of the clues in 1 down, then C in one of the clues in 8 across, D in 2 down etc.
It now seems that I have this completely wrong! It should be A in the first half of 7 across, B in the second of 8 across, C in the first half of 10 across etc.
Could Ged or others confirm this? Then I'll have another bash. If so I must say that the instructions were ambiguous. Maskerade always gives tough clues but this is a step too far! Thank goodness that AZED is a Jigsaw which is always good fun!