More oipcs
Errrm, OK yeah, these are pics of the DPM. I ummed and ahhhed about how to tank the pit, but to be honest, after a massive thaw and then loads of rain the pit had nothing in it except surface water, and digging down a couple of inches it was dry, so I've just gone for a DPM. If it really has water table issues, I'll tank it internally.
I did go belt and braces though. I taped it, sikaflexed it, and then taped it, and made sure the joins were has high as possible
EDIT: being on first name terms with your vicar has "benefits" - like using the church hall to lay out sheets of DPM that are 4m wide