Corrupted MemDisc kills startup

On my Pi I have Memphis 3 installed to reduce wear and tear on the SD card, especially from NetSurf. There’s an occasional, but serious problem. I’ve set Memphis to save/load the ‘disc’ on machine startup/shutdown, and when the MemDisc file does get corrupted, the result is a locked Pi. Startup freezes with the hourglass running over the pre-desktop splash screen.

The first two times I wasn’t aware of the cause, so I completely reinstalled RISC OS. Time-consuming, and reloading all my programs and files from backup took a while, too. Since then I’ve found that pressing Escape at exactly the right point (usually takes several tries) results in a bare desktop, allowing me to get in and delete MemDisc, then restart.

I’d like to be able to interrupt startup more easily, but I’m not quite certain of the right place to break into the Boot sequence. Or exactly what code to use. Any ideas?