I have a glow worm 100e compact. The hot water runs cold when fully on?

When you back off the tap say 30% on it runs warm to hot but the flow rate is slower obviously and it takes longer to fill the bath. Showers are out.
I've checked the diaphram - ok.
I've replaced the thermistor and now the divertor switch. however the divertor valve mechanism doesn't seem to pop out as the manual describes. Is the spring faulty or bent (it doesn't look it)? Could there be sludge of sh*t in the vacinity? Please could you help?