I decided to change my thermostat, (all tho I now wish I took this advice, If it aint broke, don't fix it)

Drained all coolant out put new thermostat in, filled it back up with coolant, and now if my car is stood still with engine running with heater on the temp gauge drops down slowly, if i start driving again it goes back up to half way, and heater is not very good at all.

Yes I know am idiot for not following advice if it aint broke dont fix it.

Hindsight is a wonderful thing.

How can I sort it.

There are 2 pipes that go to heater matrix, 1 inlet and 1 outlet, the inlet seems to be getting hot but outlet not very warm at all, I don't really want to take matrix out, as that is a very big job, and I need my car for work, so don't want to take dash out.

start with a good engine and rad flush. empty it all out again then refill the system it sounds like there is air in there somewhere. start topping with water and coolant but before it's full start the engine to get the coolant flowing

Put your old thermostat back in and return the new one as it sounds as if you have bought a faulty unit.

Good advice; certainly worth a try. Does sound like an airlock.

Disconnect the heater matrix and connect a hosepipe to the out flow pipe and back flush the matrix
this should remove any crud thats been accumulation over the years.

Reconnect everything and slowly fill it back up. There should be a bleed screw somewhere so loosen this and run the engine till hot with the car blower on fully hot.

wait for it to cool then remove the cap and top up

Keep doing this till the level settles and you have heat inside the car.

Hope this makes sense!

Steve

As the above post say's you have an air lock, find your bleed screw(usually on top of the rad) loosen it off and run the engine and keep topping up the water till it settles.
that should fix it.
Beanie.

Mine does not have bleed screaw.

I think it was an airlock, as coolant has now dropped, and I have topped it back up, heater is now lovely and toasty hot.

What was your reasons for changing a working thermostat? Seems a strange precautionary measure.