RISC OS on small TV monitor

Apologies if this has been covered before – I couldn’t find anything relevant.
I’m using a Raspberry Pi with RISC OS on a small TV monitor, 7 inch, 480×234 native.
With a default config.txt it starts up OK, and with the Acorn AKF60 MDF, 480×324@75Hz I get a fairly usable screen. However I wanted to try and find a screen that matched the native resolution, and maybe one exactly double that.
When I used an MDF with a 480×234 setting, I got a picture about half the screen height, vertically centred.
I went back to the config.txt and set mode to PAL, aspect to 16:9. Now the initial boot screen is full size, which is an improvement. However any RISC OS mode I try scales the mouse movement. It stops a small distance short of the top and bottom of the screen, and in between isn’t accurately positioned on the screen, making hitting button a bit difficult.
I have tried setting overscan top and bottom, but that reduces the height of everything, including the mouse movement.
I tried NTSC instead of PAL. This fixed the vertical scaling, but now I get horizontal scaling instead. The mouse travel is clipped significantly inside the sides of the picture.
Changing the MDF to the 480×234 one gives massive vertical clipping (about 40% of screen height) but fixes the horizontal clipping.
Switching between various Acorn MDFs I either get vertical or horizontal clipping. Auto gives horizontal clipping.
I can’t work out where to go from here. Any ideas?

Are you using the latest development ROM or RC12/12a? IIRC there are a couple of bugs with the mouse overlay scaling/positioning in the 12/12a build which have been fixed in the latest development ROM.