Baz wrote:

His garage roof drains directly on to mine, no gutters between and my roof
drains into a gutter at one end.

My problem is that he has had a leaking overflow for at least 4 weeks now
and I have asked him to fix the leak and reminded him that he said (a couple
of years ago) that he would put a gutter on his bit between us.

So if I understand you right, his leak drips on to his garage roof, then
on to you garage roof, then into your gutter and down your drain?

If so, it doesn't really sound like an earth-shattering problem over
which it's worth damaging relations with your neighbour?

David