Theres always this if the worst comes to the worst.http://www.ro-man.com/shop/product_info.php/cPath/286_47_232/products_id/61
Does it leak without the filter? Could be the filter isn't seated properly and is pressingagainst the lid.
Now why didnt I think of PTFE Im off up to PF on Friday to get my vanmount fitted, might take the offending item with me.
Did your DIY van mount fall over and land on it by any chance?:0Sorry,... couldn't resist!! SECURE THOSE BARRELS!
try ptfe tape, there could be a bit of dirt a shard of plastic stopping it seal correctly and see if it still happens if it does, I am sure they will change it for you, they have always been good to me in the past.Ian