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just to clarify, all I've done is move the hot water cylinder from the
landing to the loft. in the loft I have built a wooden platform and sat
the cold header tank (that was always in the loft) on to the platform.
the hot water cylinder sits on the joists (on a think piece of wood to
spread the load over 6 joists) and the bottom of the cold header is now
just above the top of the hot water cylinder.

Which is what I had thought you had done in the first place before you
quoted figures that appeared to say different.

the head of water above the top of the hot water cylinder must be
around18 inches (essentially this is the height of the cold water tank
since the water in the cold header is only a few inches from the top
when full). the head of water for the hot bath tap must now be
something like 8 feet (to the top of the water in the cold header)

I'm still unsure if this vast difference in the head of water is the
cause of the problem or not. all the plumbing is 22mm, right the way to
the bath tap.

It is. What you have to remember is that the vent pipe splits the run
from header tank to tap so that if the resistence to flow is greater in
the first part of the run the vent pipe allows the water in the second
part of the run to escape quicker than it is replenished. The crucial
point is not actually the top of the cylinder but the junction with the
vent pipe. I think the bath tap, even when fully open, will restrict
flow to a certain extent but, other things being equal, you need the
same head between cold water level and junction as between junction and
tap.

So you have got at the most 18 inches of head driving the water into the
tank and perhaps 9 foot of head sucking the water out. If your header
tank previously sat on the joints you should at least be getting a
greater flow of water at the hot tap than you had before.

Do you have room in the loft to raise the header tank further? That too
will improve matters.

--
Roger Chapman