Just an update... the SCART to HDMI converter arrived today!
At first I was disappointed: it just gave a white screen on both of the monitors I have here. But a couple of YouTube videos helped. First was a guy in Germany who had the same problem - it turned out to be his SCART cable. Second was one of Jan Beta's electronics videos, where he mods one of these converters to add composite video/audio inputs (...super easy, it turns out).
So tonight I fed the composite output of my A520 modulator to the HDMI converter... and got a picture! Fuzzy as expected, but shows that the converter can actually produce a picture. So my SCART cable is almost certainly the problem, and I'll be happy enough with composite till a new cable arrives