Aqualisa power shower temperature problem

I Moved into a house, where the main bathroom has an Aqualisa power shower fitted. The pump is a 654DE 3bar pump. The problem I have is that while pressure is not a problem(its too much really), temperature is a problem. Its not hot enough.

If I turn off the power to the pump, so that the water is just gravity fed, (from the hot water tank which is on the same floor, and the cold water tank in the loft, a floor above), then the temperature of the water is not a problem. Having the selector at the 1 o'clock position, (if goes from 9 to 3 via 12 o'clock) provides the right temperature. On 2 o'clock its too hot! Turning the electricity back on to the pump and even with the selector at 3 o'clock the water is just warm.

So is the pump at fault? I would guess that it was installed around 1997. I have looked at the installation and its as per the manual, with the connection to the hot tank and cold tank, and 22mm pipe work throughout.

It must be the pump, as the temperature is ok when gravity fed. (pump pulling too much on the cold side?)
thanks

Is your cylinder large enough to cope with the demand

O yes. Turning on the shower without the pump on, and you can shower completely (say 10 mins), in lovely hot water. Even with the pump powered up, the temperature of the water remains the same. It is not the case that its ok to begin with and then cools. It just cool to start with, and remains cool.

The other shower in the en-suit is also a power shower, fed by the same cold tank and hot tank. This is a Mira/New Team 1.5 bar pump, and I have no problems with this shower and it temperature or force of water.

No quick solutions so time for a call to Aqualisa I think. They have a reputation of being VERY good, but bear in mind that the cartridge (£90 ish?) might need replacing. You may even have the wrong one for a pumped shower , which may have started off as being fed by gravity. Even if it's the right one, they don't last forever.
(They're different colours to tell them apart)

Pump given up totally now. Even when the mixer tap is off and no water is flowing, the pump is pumping water. (or trying too). So I think a replacement is in order now.