Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
Does the temp dial show 90deg +/- a few degrees once the car is warmed up?
If no, you probably still have a duff thermostat. But has the cooling system been bled since the new thermostat was fitted? If not there may be an airlock preventing proper flow.
Bleed air from the screw on the metal tube across the front of the engine (open the screw until coolant runs out, then tighten), then run the engine until it warms up enough for the top hose to get warm, with the header tank cap removed and the heater turned on. Squeezing hoses by hand can help to dislodge trapped air. Turn off, replace cap.