Thanks Zip, I've just filled it up again very slowly and I can't see any leaks from that bolt/spout/underneath, So hopefully it's exactly what you've said and I won't need to touch that bolt.

I don't actually have a dipstick, guess I'll have to buy one.

With the cooler lines, I've got one attached now, seems like I broke off the copper pipe in the correct places before, so just needed to undo the end fitting, first one was tough but managed it, second bigger one is not budging, guess i'll have to spray something on there to help loosen.