Shower problem

Just fitted an Aqualisa thermostatic mixer shower. All the pipework is in place as stated in the installation instructions, hot left, cold right, mixed water to shower head on top etc.... The cold is connected to mains as instructed, and hot to combi boiler. The instructions say to turn temp control fully anticlockwise, ie. cold, and turn shower on to check for leaks. Done this and no leaks its all ok, but the flow was abismal!!! The combi isnt live yet, could this be the problem, and should it work ok when the boiler has been commisioned and fired up? Thanks for your help.

Put it to cold water only and see how the flow is. You will know for sure if it is ok when the combi is working.
If you have piped it properly then you may be worrying unduly unless of course the mixer is faulty.

I tried the cold only and it was just a trickle. I disconnected the cold supply to the mixer and turned the supply on directed into a bucket and it was fine, lots of pressure like you would expect like a cold tap!
Could it be because there has to be hot water as well to open the valve or something?

What model valve is it?

Did you thoroughly flush the pipework before fitting the valve, or could there be crud blocking the inlet strainer(s).
Did you fit a flow restrictor into the cold inlet, and is it the right way around?