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.
Nicholas John Peter Tinker - # 484
Manufactured on Tuesday 27th April 2004 @ 12.17.47.
Commission Number: 24468.
Brochure Model: M3 Tourer 4.6 V8 260PS.
Specification Code: L01B.
CICode: D1511.
Trim Level: TL8.9 (M3).
65th ZT-T 260SE to be made out of 115.
11th ZT-T 260 SE in Pearl Black (PBT) out of 23 produced. Manufactured in the UK by MG Rover Group Ltd.