As far as I can tell it's an expansion issue, rather than anything slipping/the door hinges dropping, and it's only a case of millimetres. Is there anything a diy-er could do to fix this?
It is a case of adjusting the hinges to give a bit of clearance.
My starting point is using a soft pencil and marking the four corners of the door, where it closes against the frame. Then adjust the hinges to get even overlap left/right and also allow a gap for expansion. Then vertical adjust top/bottom of hinge side. Level across the top is then set by heel and toeing glass.
If it's not obvious what needs turning, get a picture of the hinge (face and consealled edge) and I'll point out what to a adjust.
Edit: now that I've posted the pictures, is it as simple as just loosening the screws in pic three and sliding the plate back slightly?
No, they adjust how tightly the door latches closed, by moving the keep closer to the seal or away from it.
Please give that locking mechanism a brush off and spray with some 3in1 oil.