I'd been wondering how I would hold the insulation between the rafters.
It
strikes me that if I get another lot of kingspan/celotex of about 20mm
thickness (if the cost doesnt prove prohibitive), then I can install that
over the top of the rafters (well, on the underside - ie on the loft-space
side)

Indeed. Doing so is strongly recommended, and verging on the compulsory for
new build/loft conversions. Keep the lower layer of insulation at or below
the thickness up the upper layer to avoid condensation issues. I would put
50mm between and 50mm underneath. Ensure that the top layer is foil covered
to give a reflective surface for radiated heat from the tiles (you'll
probably find that the boards come fully foiled anyway).

Note that you should cover the entire lot in 12.5mm plasterboard. This not
only provides an aesthetically pleased surface, but has other benefits, such
as making the space lighter and providing much needed fire protection to the
insulation.

Are there any vapour-condensation issues for cold roof rafters so-covered?

Your proposed solution is OK for condensation, provided the roof timbers are
ventilated to the cold space. This is why you must leave the 50mm gap. If
you had no membrane, or it was breathable, you could fully fill the cavity
and assume that the timber is still ventilated through its upper surface to
the tiles/slates.

Christian.