Thanks for the replies,
Not sure if it's a safety feature but our mixer won't do hot only - it seems to mix cold with it. However, I tried the "turning it to full hot and lowering the shower head".
Result - no spluttering, just powered cold water.
As I said you can have a perfect temperature shower as long as you don't want it powered. As soon as you throw the switch for the pump - it turns cold.

Any more thoughts?

Thanks
Jonathan