RC15 fails to boot on RPi B+

I got a hankering to play in RiscOS and/or BBC BASIC again, and decided I’d like to run it on real hardware for once as previously I’d used an emulator, so I bought myself an RPi B+, specifically because I’d read here and in other places that it was the most compatible with existing RISC OS software.

Trouble is it doesn’t even seem to be compatible with RISC OS itself …

I’m unable to get either RC15 or the Pico images to boot. RC15 boots to a black and white logo, and then does nothing. Pico boots up to a large blinking cursor and also otherwise does squat. I even tried swapping out the RC14 ROM and that does the same as RC15 only the logo’s all squatty because it’s clearly not detecting the widescreen. Installing from NOOBS instead just doesn’t give me a video signal at all once it hands over to RISCOS to boot.

I thought it might be power issues, but even with a 2.4A PSU no form of RISC OS will boot for me, though I am able to get Raspbian installed and running just fine (for certain degrees of fine; it is not what I’d call fast).

Is there some config.txt incantation I’m missing or custom patch I need to get it running?