If that was the case I'm surprised that turning on a cold tap (even at
a low flow rate) upsets the shower temperature so much. This should
have negligable effect on the hot supply.

I was suggesting that there were two effects here, due to a postulating
hypersenstivity of the shower mixer to pressure.

1. The hot pressure, being gravity fed would be affected by the height of
the shower head.
2. The cold pressure, being mains fed with a constriction on the house feed,
would be affects by other outlets in the house being turned on/off.

However, saying all this, we are going to ditch the venturi shower as
we are going to a combi-boiler in a few weeks.

You may still have the problem with the cold pressure being variable.

Christian.