Mixer shower - Check valves?

I am fitting out a new shower room and am in the process of running the pipework. The shower is to be fed hot water from an existing hot water tank and the cold is to be a direct feed from the cold water storage tank. The thermostatic shower mixer appeared to have a form of check valve on both the hot and cold water inlet. I completed the pipework to test stage and for the hot supply and part completed the cold supply but did not connect this to the supply tank. I opened the gate valve to allow the hot water into the system without problem but then noticed that the hot water was back-flowing along the cold supply pipe! I had assumed that the check valve inside the shower mixer would not let this happen so I am a little puzzled.
As both the hot and cold will be at the same pressure when completed will this stop the back flow when the pressures at the mixer valve are equalized ?
Should I fit a supplementary check valve on both supplies

you can try it with the cold connected. but it shouldn't be letting any water backflow at any time even with the cold not connected

Ditto, the hot should not be flowing up the cold. I say your shower's b*ggered, and as it's new you're perfectly entitled to return it for replacement.

Thanks for the advice. I shall contact the seller to see if I can have it replaced.

And assuming it has a CE or kite mark the hot shouldn't work if there's no cold.

Should say did you flush the pipework out.