SITUATION. Systems all checked:

Pipes flushed, a sparky has checked electrics, new combi boiler (£7.5k ouch) and providing water to required temperature, thermostat calls for heat, a new actuator (replaced 1 week ago), mixing valve checked, pump checked (set at III or II), flow meters showing 1.5, temperature showing 32 degrees.

PROBLEM. Water supply pipes up to manifolds hot but tepid once in manifold. Floor tiles not warming (chill taken off). I believe too much cold water being mixed.

QUESTION. How do I increase temperature of water going under tiles?

Is the room up to temperature? And how how have you given it to heat up?

UFH doesn't or shouldn't have a hot floor. 32 degs is below body temperature so it will only feel tempid.

That 32 degrees on the manifold is the clue for me. I would check the blending valve on the manifold first and nudge it up in small steps, as it sounds like it is mixing in too much return water before the loops. Also compare the flow and return temperatures on a loop and make sure the actuators are actually lifting, because a stuck pin will leave a loop cold even when the pump is running. Tiles will still only feel mildly warm at normal UFH temperatures, so give it a good few hours after any change before judging it.