It's a hudson reed (A3099)gravity fed twin thermostatic shower

When I turn the shower on I get a dribble when it's on the hottest setting and a bit more of a dribble when it's on the coldest setting. If I turn the shower to the hottest setting and leave it on and then go downstairs and turn on the hot tap in the kitchen for 5 seconds or so the shower will then start to work fine but only on the hottest setting, can't get cold to work. The second I turn the shower off and back on it won't work again until the hot tap in the kitchen is switched back on for a few seconds. But I can't seem to get any cold through it other than a dribble. 

Cheers

I would suggest replacing the entire guts of the fuacet (hopefully only one cartrige). See if that helps.

Ok, thank you, I was expecting it to be a cartridge fault. Will try that first and go from there.

I have isolated the cold and hot water individually and removed the thermostatic cartridge. I have then turned on only the hot supply, it gushes out of where the cartridge would have been, but, when I put the cartridge back in I have to switch the kitchen tap on then off to get it to work properly.

I then isolated the hot supply again and turned on the cold supply, this only dribbled out of the hole where the cartridge should be. I removed the chrome elbow feeding the shower unit an removed the one way valve thingy and cleaned out the gauze in the rubber seal, refitted and tried again, slightly more pressure but not as it should be. There is no problem with cold pressure anywhere else other than to the shower. Any ideas?

How is the pressure on the cold with the elbow removed (not hooked to faucet?). It may be something in the shut off, or piping. Need to back track to see where you are losing water. Most newer shower faucets, they have a balancing valve in the cartridge assembly, so if you aren't getting good pressure from hot and cold, you won't get good flow from either out of the fuacet. So basically, you need to find where you are losing the cold pressure/volume.

I have now fitted a new cold water feed straight from the mains supply, I have also replaced the thermostatic cartridge, when the cartridge is removed I now get hot and cold water flow from the faucet, albeit with higher cold pressure than hot. I have temporarily reduced the pressure by practically closing the mains stop cock so both pressures are equal. When the cartridge is in, cold will flow through fine but I still have to switch the hot tap on then off for it to flow through hot. I'm wondering if this is because the shower head is too high for the gravity feed to work, but the faucet is OK? Although the head is actually below the hot water tank. I have mains water pressure feeding the cold water all over the house but the hot is gravity fed. If this is the cause would a shower pump over come this problem for the hot water to the shower? What sort of pressure pump should I be looking at, I had the salamander CT85 or the salamander RGP120 in mind, but I have 22mm hot water piping, I believe the CT85 only comes with 15mm, I know you can get reducers but is this worth it?

Thanks, Rob