Hi Chris, welcome along. Your leak is probably coming from the distribution valve which directs coolant to the dual zone climate control. It is the same valve that is in the N/S plenum under the scuttle panel and is a well known problem. The one in the plenum is replaceable with an upgraded valve from the club shop but I don't think it will fit as a replacement for the one attached to the heater matrix. Some members have replaced this with a simple three way 15mm copper T piece with short 15mm copper pipe stubs with olives soldered. This resolves the problem but you will no longer have dual zone CC. I'll see if I can find any more info.

Nich.