How can I fix my Aqualisa Classic Aquavalve 400 manual mixershower?

Hi, I have an Aqualisa Classic Aquavalve 400 manual mixershower, installed in 1990. Water is fed to the shower from a hot water tank in the bedroom next to the bathroom. The pipes go up from the tank into the loft, across and down to the shower in the bathroom.

When the shower head is laid down in the bath, the water flows through fine, at a reasonable pressure and hot.

When the shower head is hung in its position on the wall the water loses some of its pressure and the goes cooler.

You cannot turn the shower on when it is hung on the wall, it just runs cold, you have to lay it in the bath below first and let the water run hot for a while before you can hang it up and use it.

Any suggestions as to why please?