The UK-RR Sten mag should work, as should your surplus mag. I just pulled a UK-RR mag off the printer and I’m seeing the same problem. Feed lips hit the slide before the mag catch can engage. The location of the mag catch in the frame seems off. With the slide off and the mag fully inserted, I measure 78mm from the bottom of the magwell to the top of the feed lips. But with my slide there is only 75.5mm from the bottom of the magwell to where the mag hits it, so I’m getting 2.5mm over-insertion. I’m seeing the same thing with three different slides (one Glock and two aftermarket) so I can’t see how this is supposed to work.

Fit in the magwell with the UK-RR mag also seems pretty sloppy. The magwell is 24.2x39.3mm. Sten mag specs I’ve seen put mag size at 23.1x39mm, so surplus shouldn’t be a bad fit (I don’t have a surplus mag to test). But the UK-RR mags are only 22.6x38.1mm, so over 1.5mm side-to-side play and 1.2mm front-to-back. I’m going to remix the UK-RR mag with slightly larger dimensions and a higher mag catch slot to see if I can get something that will at least feed.