I thought I'd hold off ordering anything until after everyone is back
from the Christmas/New Year break, so this morning I enquired at two
separate places this morning, the local family-run bathroom supplier;
and Plumb Center in Spalding.

Both told me the same thing independently of each other: Water regs
prevent swapping out two separate taps for a mixer tap/shower where
the cold water is at mains pressure, which mine is. Apparently, once
the mixer is installed, when the cold and hot are turned on, the high
pressure cold water will overwhelm the gravity-fed hot.

Two solutions: One is to fit a pressure reducing valve in the cold
feed, but the Plumb Center chap said this would have to be screwed
right down to minimise the pressure.

Better (both suggested this independently): Run a new length of copper
pipe from the cold water tank in the loft to the bath and cap off the
existing mains-pressure cold supply to the bath. NB: The cold water
tank currently only replenishes the copper cylinder in the airing
cupboard. Disadvantage: The new gravity-fed cold feed to the bath
would mean that the bath would take ages to fill. Not sure how much of
a disadvantage that is, although I will say that it DID take ages to
fill a decent bathful at a relative's house over Christmas. They have
gravity-feed. Only mains pressure to the kitchen cold tap.

Finally, the Plumb Center bod was very helpful, spending a good five
minutes going through the options. he said one thing I'd need to be
aware of: Getting the old taps off is a dickens of a job as one has
only minimal space on the underside of the taps.

MM