I have an 20 year old combi boiler and a bar (Bristan) shower control. I have good water pressure through all taps but cannot balance the cold/hot pressure on the shower without losing cold pressure in bathroom basin. To get a warm/hot shower the feed to these and the w.c is a mere trickle as I have to reduce the inflow of cold water

Knowing which model you have would narrow your search for the cause and the correct part.

Bristan mixer showers in a wide range of designs to suite your bathroom (http://www.bristan.com/Showers/Mixer)

I am almost sure its a valve/cartridge problem.

http://www.bristan.com/Customer-Service/Troubleshooting

Pictures are helpful also.

Do you know the diameter of your supply lines to the basin and the shower? This does not sound like a pressure issue to me, its more of a volume issue.