Triton Bezique ll not heating water?

I have a Triton Bezique ll installed some time ago and in the past week it now is taking up to 5 minutes to get the hot water through. When I push the button for full heat, the water pressure light and the cold water light come on. After a while the hot water starts to heat up and sometime later the full heat light gradually displays and the other 2 lights extinguish. Triton Services say it is low water pressure but the dial is set on 6 as it has for most of the winter.