Help!!

[Drew84]

Hi, i hope someone can help!
I have a Mira PS1 shower pump located in a kitchen cupboard in my appartment. There is a cold and hot water feed down to my bathroom to a gainsborough shower. The pump only seems to work when the hot tap is turned on in the bathroom. The pump has (i believe) a pressure switch on it, but it may be a switch of some other type. I do get hot water out of the shower, but it gets there much quicker with the hot tap on. Once it's on the temp is all over the place, and seems to just get hotter and hotter. Basically, i need a bit of advice on how it may be fixed, and whether i can change the shower unit, and what type to get? It looks fairly simple- unit in middle, with adjusting dial on it, with a pipe out of either side, and one out of the bottom.
I hope someone can decipher what i mean, and any help would be greatly appreciated.

 

[mattylad]

Pictures say a thousand words

 

[TicklyT]

Shower pumps are often controlled by flow switches. If there is insufficient flow from the gravity feed, the pump doesn't start. It sounds like opening the hot tap increases the flow enough to start the pump

Clean any limescale from the shower head. a gradual build up can reduce the flow rate.

Assuming the shower head is on a flexible hose, drop the head down into the shower tray/bath and turn it on, and see if the pump starts.

You could remove the head from the hose, and see if the pump runs.

The flexible hose can get twisted, and collapse internally, reducing the flow.

Reduced water flow could make the temperature control over-sensitive.

If the head is bunged up with limescale, there's a chance the mixing valve may be too