Overheating?

[Arg1211]

Hi, new to the forum, looking for some advice. I have just picked up a 2001 berlingo 1.9d last week and unfortunately already have a couple of issues which I hope somebody may be able to help with.

Firstly on cold start the engine is quite loud, like a knocking noise but seems to settle after 30-60 seconds. (Maybe normal for a diesel of this age although this is my first diesel vehicle so unsure).

Secondly, sometimes on start-up the temperature gauge moves straight to the max and STOP warning light flashes on dashboard, also radiator fan starts up. After leaving 10 minutes the van will start without the temperature issue. Whilst driving the van doesn't seem to get up to temperature and stays in the lower quarter. I would guess at a dodgy thermostat, although unsure where to find it to replace?

Thanks.