[SpecB]

Water pumps are fairly reliable - Thermostats are designed to fail in the open position so shouldn't cause over heating but could cause slow warm up.

I'd start by dropping the coolant and then refilling using the Bottom up method to try to eliminate airlocks (Water would be fine for a bit as a test).

Do you get any bubbles in the Overflow tank and is it filling excessively when hot if so the usual culprit is a HG failure allowing exhaust gasses (combustion chamber) into the coolant it's quite rare that the coolant and oil mix in the flat 4.


Bottom Up coolant re-filling method.

1. Drain from the rad drain plug, or remove bottom hose. Set heaters in car to hot while it drains, leave them there.
2. Inspect rad for the core turning to dust, if it's bad, replace it, internet ones are fine.
3. Use a decent quality anti-freeze, as at a motor factor for one suitable for an alloy engine and having anti-corrosion added. Dilution is usually 50/50.
4. Replace plug or bottom hose!!!
5. Remove the rearmost pipe from the metal header tank on top of thengine, remove the cap from said tank. Jam a funnel down that hose and fill SLOWLY via the funnel, do NOT allow it to gulp.
6. Watch for the header tank to fill, once it reaches the top, remove funnel and replace that pipe.
7. Now, with cap still off, start engine and allow to warm up until fan comes on. Watch for bubbles, top up or mop up as necessary.
8. Once fans start, switch off, replace cap, drive round block, leave to cool.
   When cool, remove cap, check level, top up if necessary, replace cap, job's a good'un.

It takes a while, but bear in mind, THERE IS NO KNOWN WAY TO BLEED THE SYSTEM IF YOU AIRLOCK IT.