Have heard back from Ilford and certainly they're not dismissing my enquiry. I'm going to submit the scans of the negatives I've posted up plus one of the unexposed sheets I developed. I did another test this morning as panic set in that perhaps I was storing the film incorrectly in my freezer.
I've got a box of Foma 400 that has been stored in the freezer for much longer and the HP5+ has been sat on top of it since I bought it a month or two ago, while the Foma is out of date I thought I'd throw a sheet through the processor and see what happened. Sure enough, the sheet of Foma was clear as day with none of this vignetting going on. So I'm pretty much convinced the film is at fault. Hopefully Ilford will see this too and send me a couple of replacement boxes. There is light at the end of the tunnel!