heater

I guessed this might be the next stage. I would always be suspicious where the thermostat has been removed/tampered with.

Its going to be a process of elimination.

The first is to look for air-locks after you have installed the thermostat.

Then run it up to temperature with the radiator cap off. Look for flow once the thermostat opens. Look for bubbles and/or oil (Head Gasket ). Look for really soft hoses (ie. no water in them) or really hard hoses (system pressurised - head gasket)

Check the radiator is not blocked externally, look for mud collected in the bottom of the ducting.

Like Martin says sounds like it could be an air lock, Run it from cold to hot with the rad cap off (not moving) as suggested, also have the heating on hot and blowing.
Carefully give any water pipes a squeeze whilst its getting up to temp too to help move any air out of the system. (careful for hot water and moving parts though!)

If that doesn't work start flushing the system out.

Is it running OK other than the temperature fluctuating? Is it losing coolant or oil?

well i just took it out drove 30 miles then i noticed the gauge started moving up and stayed on the half way mark and was all toasty then i stopped for 2 hours started up again and it was fluctuating between half way and cold and was blowin luke warm air so ill bleed it out tomrrow see how that goes yea no other probs apart from thati hope its simple just want a warm car

so i bled it all out, now since i got my new cap its been holding pressure and the heaters are blowin warm air now and after 20 miles it goes up half way better than over heatin so thats good i fink ill have to check thermostat again i have a feelin ill need another new one due to fiddling and airlocked and it goin from hot to cold etc etc i fink thats why the thermno was in wrong way people have been tryin to figure out whats wrong and all it was was the rad cap and i know it works now as took it off and sprayed loads where as before i was drivin 65 miles before and i could take cap off and nuffin happened so will update on how thermo is and i hopefully touch wood will have sorted it atleast i have warm air now lol better than freezin thanks for ur help everyone cant wait till its proper sorted i have just put a new bumper on as well today as old one was hangin off and snapped on side my fault goin to quick round a roundabout also have alloys, bullbar, sidesteps coming 2 and then in new year a new top as this ones had it really!!! ill put picis up once it all arrives:laugh:

good stuff, sounds like its sorted then.