Following the post from edam above, I purchased a metre of 1.5m wide black craft felt from fleabay (item 310938609603 ) and a roll of non-slip mat from Wilko (1.5m x 50cm for £5). A bit tricky to get the shape absolutely right but using masking tape and patience and it's a good fit. Cut the felt, masked the edges, sprayed the back of the felt with foam contact adhesive (3M type 74, others available) and laid the non-slip mat on. Note I didn't spray the non-slip mat because with the holes it would go through to the side which would sit on the dash panel.
Excellent result, the difference is the proverbial night and day. Almost no reflection at all from the felt, well worth the effort.
You could get away with doing a much smaller area than I did, but covering most of the top means it doesn't immediately look like an afterthought. Just needs a slight trim here and there, I'll reduce it a bit just to the left of the centre vents so it sits flatter, and some holes for the screen vents.