Low Hot Water Pressure - Causing Machine To Fail

Hi there, I've been having problems with my Zanussi washing machine recently, I assumed there was a mechanical fault of some kind but it appears to be fine, the problem seems to be caused by the fact that the hot water fill is not high enough pressure.

The machine runs its cycle fine until it gets to the stage where it wants hot water. It then sits there with the hot fill valve running (making a buzzing noise) until I move it on manually - but then the wash is poor. The valve itself is fine as I tried connecting the cold pipe to it then it works perfectly so there appears to be a problem with the water pressure which I assume I cannot do much about, the machine is fed directly from the HW tank but there is no pump.

How do I get around this problem? Can the machine be used with cold water only? I thought the reason it was after hot water was perhaps a fault with the heating element or thermostat but I have tested both and they're fine. If so, how do I plumb it in, because if nothing is going in through the hot inlet, it will just sit there infinitely waiting for HW?

Many thanks for any advice.

Regards,

Gary