The OEM thermostat is still seems to be running better, temp still dips a little but is a lot more consistent. There's a few little hickups that crop up under different throttle loads, but I'm hoping the new manual ECU should resolved those. 
 
Now onto the fun and games while I work out whether my SCer or water pump is squeaking and rattling. I'll probably just keep running it until which ever it is dies now then chuck in a spare.
 
Thanks again!

Ahh so you got the pulley sorted and have been driving it around?
Did you end up finding the manual ecu?

Yeah took me a couple of hours to copy the old pulley guide and used an old 3s cam belt pulley. Finally had it running longer than around the block and experienced the infamous interwarmer's effects. With the belt really tight it sounds ok but theres a bit of a bearing rattle/squeek which I'm assuming is the SCer but I'm rebuilding a spare at the moment so it shouldn't be a drama. 
 
My manual ECU arrives Friday so fingers crossed its all ok and might smooth things out even more.
 
Over all its running pretty well, an absolute dog if its sitting at the lights but gets going after that and a little bit rough through 2,000-3,000 while its cold but its no where near as bad as it was.
 
Back to rebuilding the suspension and fixing a few other body things then its all sorted!

How is the water pump ? - I have seen these with the impellers rusted off, but the bearing is still ok so no audible sound. If you have lost a blade or 2 then that may upset the flow. the Temp sensor (gauge) is up high on the engine after the water pump, if it's surging then you may have no water over the sensor at times - this would give a low reading.. they only read high if coolant is over it. I would wait till it's cold, remove the filler cap and start it - when the thermostat opens you should see the coolant flowing towards the front in a smooth motion. if it splutters and carries on then just replace the water pump....