Yep, good idea. I'll hopefully get to do that over the weekend. I'm currently boarding out the extension so like to keep the floor off during the day (as it sweltering otherwise) but I want to test a few things this weekend and try and get to the bottom of it.While you are waiting maybe try ShaunCorbs suggestion first and if no joy increase the ABV setting to say 0.25/0.3bar and note the UFH flow gauge readings before/after any changes, you can always change it back after your tests.
Thanks for your help. I'll post back with any developments