Right so I've made a bit of progress it seems. I didn't have a dremel or anything around (all in the shed) so I decided to use a box cutter as a replacement. Instead of dremeling, I just sunk the blade into the polymer which I wanted to remove. There were three tiny teeth sort of things in the mag catch hole on the pmag. I removed all three of those, and slightly increased the height of the hole by about 1mm (probably just under) by rubbing the blade in a line along the polymer. Gotta make do with what you got, right? Anyway, after doing that and being amazed at how clean it turned out, I popped the mag in. Now it didn't catch properly, but I pushed the mag release thing (not the button, the bit on the other side) down (as if I was trying to push the latch into the hole of the magazine). Well, that seemed to work and the mag isn't coming out I will increase the height of the hole by like a quarter of a millimetre (hopefully) and see if that just allows the mag to slide in without me having to push that mag release bit.
That probably made no sense and was poorly described, but I will try provide pictures once done.
Yeah that seemed to work, although it won't just go in like a normal mag, you have to slap it in as Qlimax suggested. I just had to modify mine first.