hot and luke warm, low pressure at shower head?

Just to add I replaced a diverter valve recently as it was leaking between the two sections, however the temp of the water varied before the replacement.

All the rads heat up well.

The shower unit is a thermastatic one. also, when the HW tap is turned on downstairs the shower stops aswell as bath tap.

The boiler is situed downstairs.

Can anyone help as having a shower is a real pain with low pressure and varying heat!!!

Ats
October 2007