Thanks BurgerKingDiamond,
Update - Changed my mind again and have replaced the back panel (itself a replacement) with a
less obtrusive narrower one.
The problem was that it would have to sit at each end on two supports and be bevelled to match
the mitres that hold the CP in place. This is fine but the errors in construction have well established
themselves by now and everything's a little off.
The Whale Cut.
The Hammerhead.
Used the router and a chisel and finished off with the mitre saw. Went OK.
Hinges next - found some lovely ones called Soss - German I think.
Hope there is enough clearance when it flips up or I will have to move the monitor up a bit.
Cheers