Aftermarket thermostats

Perhaps that's what it is - a stuck open thermostat that lets the coolant circulate all the time. I think you have no choice but to remove it anyway. It's possible something (like a piece of silicone) got stuck there and keeps it open. Cleaning it might solve the problem if you're lucky. It's a simple working gadget.

I'd be dubious about it running cold Gavin, are you sure the temp. gauge is reading correctly? If the fan comes on at standstill then I think you can rule out running too cool. The DFIs seem to run rich in the UK, I checked everything (all checked out) & ended up putting a resistor in parallel with the air temp. sensor to offset it's reading. I get nice tan brown coloured sparkplugs now.