Clean water tank leaking - Lid TCL/4?

Hi,
I have a wet patch behind the drivers seat after a weekend away. Looking around the black clean water tank, I cant see anything obvious, but have discovered the TCL/4 lid does not have its seal (p/n AK4010).
I also find its easy to catch the thread incorrectly.
Any advice on where these tanks tend to leak, where I might buy an AK4010, advice on removing the tank and any other advice?!?!

Filler pipe?

I don't think its the filler. I will try and re-fill and monitor when the weather is dry and warm.
the only other thing I noticed was the high level of condensation at the back of the tank (closest to the outer wall).
Has anyone seen high levels of condensation mimicking a leak?

Hi Rick , I could see  a tank filled with cold water having condensation on the side next to the outer wall where the temp is cooler and less air movement, especially in the cold humid air experienced over the past few weeks.If there is a leak somewhere in the van the moist air produced  will condense on the outside of the tank but it may not be the root cause only an indication of some other fault. Forgot to say if your seal is missing and lid cross threaded and not pulling down square to the tank then driving about could cause water sloshing about the tank to leak easily, replace that first and retry.