I have the same problem on my 97 GM low blow 1.7 td. I do know that the head that was fitted did have a very slight crack forming across one of the ports. Starts first turn of the key from cold but a slight splutter for a few secs but after that it runs well but it is a tad down on power. It does lose a fraction of water but is very inconsistent. Sometimes the level doesn’t move from one week to the other & other times a top up is needed after a week. There does seem to be too much pressure in the cooling system – even if it’s been left all day, if the level looks low and I come to remove the cap you can feel a lot of pressure releasing and forces the water back up to the max mark again.

Thanks railroad, I'm keeping an eye out on ebay for a cheap head (local breakers seem to want more money that the old vans worth but if I can't drop on one cheap will give that motor seal a try. I said I'd scrap it once it reached 200k miles but its almost reached that now and still plods on well & good mpg so might spend a few quid on the ol gal to keep her moving along.