Trauma Water Filter Housing Leak

My caravan has a  Truma water filter housing which requires the filter to be changed periodically. When I initially took delivery of the caravan the filter needed to be fitted which I did for the first trip out. Although it is a simple job to fit I found it very difficult to stop a small leak from under the filter retaining cap although I did managed to do this by tightening the cap VERY tight. It only leaks whilst the pum is running. When I replaced the filter the leak has become worse and no amount of tightening will stop it, no matter how tight I screw the cap on. Has anyone else had this problem and, if so, was there a "cure"?

The cap has a round section "O" ring in it but, in my opinion it seems far too small in section. A flat section ring would, I think, have been a better design.

I've spoken with Truma who were unable to suggest why it leaks but have suggested a replacement housing under warranty but I may not have time to get this done before my next trip.