What a bear of a job!
The matrix wasn't leaking...just spillage from the valve. while I was at it I also took out the main radiator for cleaning. Filled both with petrol and steeped for an hour, then emptied out half the petrol then gave them a good shaking...repeated process twice. Then cleaned with fresh water. The main radiator was quite clean, but the heater matrix was restricted...probably 30% flow, which would explain a poor heater! All built back up with fresh anti freeze, heater now working fine.
Thanks.