Im using this to drain water from my sink. http://www.biltema.se/sv/Fritid/Husvagn-och-husbil/Vatten-och-avlopp/Spillvattenlunga-37109/ It works well and holds enough water with regards to my Helsinki tank. Im not sure if they will send to the UK but I wont mind sending some over.
we are allowed to just "let it go"... Mine has a big tank ,come to think of it I have never emptied it..