Hi, yes I noticed a leak on mine the other day in exactly the same place. I've had good service out of it (240,000 miles). Like you, I would prefer a brass replacement (repairable) but don't know if I can get one in the UK. I suspect that when it starts leaking, it's kaput. Replacing expansion tank cap is probably a good idea as well.

Krolroger
230TE 1989