Thanks all for the replies - Arne in particular- I have read your work avidly. The mast you have drawn is much taller then the existing mast on the coromandel but the existing mast seems to be built of 2 sections, with the upper section tapered and the lower straight. So it could possibly be sleeved into a straight pipe at much lower cost and easier availability than a tapered pole.
The coromandel is really very sorry for itself otherwise I would consider it - and my tastes run a bit more folkboat...
The lack of a sliding hatch makes the headroom a bit low, but it's hard to keep a sliding hatch watertight in a small boat in a seaway - I am Irish Sea based so the sea can get a bit lumpy...
I will contact the owners and possibly take some measurements off the spar
Best
Charles