Hi

My (very) old Vaillant seems to have sprung a leak. When the hot water
is running there's a leak at a nut that drips about once every 10
seconds. Here's a pic: http://warren.blackbeansource.co.uk/leak.jpg

I've tried tightening the nut very slightly but it doesn't seem to
make a difference. I assume it's a washer inside that's damaged or
something. Is this something that I (a non-plumber!) can try to fix or
do I need to get a professional in, and do they need to be Corgi
registered to look at this bit?

Thanks
Warren