How to release an awkward screw on the PRD on Triton Jade II?

Hi, the pressure release device went on our electric shower. It is located on the left hand side of the can, about 1/2" from the left hand casing. There are two screws holding it in, I managed to get the near screw out, but I can't get the one behind.
I have tried pusing a small screwdriver bit in there and rotating it with a spanner, but it is still at too much of an angle. I can't see any way to remove that side of the casing.
Any ideas much appreciated.