I have looked up on web and come up with a Wirlwind xl being an Eldis, as I ruled out it being a vaccum cleaner, washing machine or fridge. The following additions to your questions would have been helpful.
1. Is it an Eldis? 2. Is the water heater a Carver? 3.Is the cold water inlet unit a Carver Crystal filter unit?
If Yes to (2) there is a none return valve on the cold feed pipe to the heater and MAY be stuck partially Closed hence the dribble. I take it the cold works fine!
First contact Eldis IF it is an Eldis. If they can't help they may give you the phone number of Carver.
Elddis Caravans
Delves Lane
Consett
Durham
DH8 7LG
01207 503477
Bob
September 2009