Diesel fill causes sender to blow off top of tank

Don't know if you got this sorted, but I had the same on mine, though luckily not whilst filling.

A previous owner had replaced the pump/sender unit but hadn't reassembled it correctly which meant the retaining ring wouldn't seat properly and would pop off.

If I remember right, it was down to the seal, when I first looked at it, it was around the 'head' of the unit and it was really tough to get it far enough into the tank to get the locking ring on at all and it wouldn't screw down very far.

It turns out this was wrong, the seal needed to be put in the neck of the tank before the 'head' of the pump/sender unit was pushed into it. Once I had done this, the retaining ring went on much easier and screwed down much further.

Hope that makes sense and helps 😊