mjk - haven't got all the theme, but am seeing part of it. I have some unclued answers that appear connected - Barbara (Windsor), leading to 'knot', 'soap' and 'chair'.
I still don't have answers for 19 A and 37 A- any advice,tips,solutions?
I have a full grid (I think) although my 5 thematics do not have any abbreviations. Is it a royal connection to the Windsors. No idea what the 21 letters lead to?
Don, the abbreviations bit didn't mean anything to me - so I just ignored it.
The 21 letters lead to the unclued answers at a. next to 6 D and b. the three-letter word ending at 23 D. You said your grid was complete - mine wasn't till I'd worked out the name to be written below the grid!