Hi. Ihave just had a(Grant) combi boiler fitted, my shower used to have a pump on it but has now been removed when the new system was fitted. If the shower is running and I turn the cold water on ,at the basin there is a small drop in pressure as you might expect but when I then turn this back off, and then turn the hot tap on it stops the shower from running more or less completely, please could you tell me why this is. Thank you C.Tarry

You have a blockage somewhere.

You may have diverter problems as well-especially if it's a cartridge type diverter. You would be best served by a competent plumber, as A. a properly plumbed home will suffer no pressure loss when operating fixtures (IF you're on city water with proper pressure) and B. he will possess the answers you seek.

Are the shutoffs to the shower diverter completely open? (does it even have shutoffs?) Are the valves open?

I would contact the installer and see if they'll assist you. Many companies are quick to respond to warranty calls, and it may be something as simple as a valve not completely opened. If you have galvanized pipes for plumbing... ewwwww... you may have some clogged pipes causing your pain that will need to be replaced with copper or PEX to resolve the balance issues. Regardless, this forum cannot help because this will be a hands on diagnosis to properly resolve your problem completely.