Thanks to All
Several good ideas to consider.
Replies to all...

NT, I do have a sample of the shape in that I have some of the
original coping blocks to work from, so we got that covered.

Steve, I think that's the way to go, but I don't completely understand
what you're saying - what you're doing with the lath and plaster of
paris. I was thinking to line up the existing blocks with shaped
plywood separators between them, wrap them in plastic (blue DPC
membrane), apply a few layers of glass fibre & resin, hold the fibre
tight to the curves with laths screwed to the spacers. Maybe another
layer of plastic first to stop the laths sticking to the mould. Maybe
a layer of stiff cardboard to help keep the shape. Sound OK?

Rod, I think the rubber would be good for small items, but would be
very expensive for this.

Kev, it wont fit in the oven - it's 3' long. I'll use the tip another
time though.

TNP, that sounds a good way. I fancy having a try with the resin
though.

Baz, I'll remember your hotair gun suggestion for future reference.

Malcolm, I had a look at the bendy MDF. As you say it wouldn't give a
small enough radius. It'd be fiddly cutting lots of extra slots, and
the pieces they had were too small in the non-bend direction. Good to
know it's there though.

Tony