If it is losing coolant one place you can check is the inlet manifold. The rubber gaskets between the top alloy part and the plastic can leak, it's quite subtle and can be hard to spot
You need to check both sides.
The alloy corrodes and distorts the gaskets, in extreme cases it can crack the plastic
Good old OAT strikes again.
I've got one sat in my kitchen at the moment have got it all cleaned up and ready to re-assemble.
If anybody wants a spare inlet manifold give me a shout.
Russ