After a couple more attempts I think I've got the fail-over power supply working, using the dc-dc switcher module. I had to modify the module in two ways:

- bring out the ENable line so I can control it

- remove the trimmer pot and replace with fixed value resistors to give ~5.5V out

- remove the 0.1uF cap on pin 8 so that the 'soft-start' feature is disabled. Without this the switcher takes ~15ms to come up, leaving a nasty gap as it takes over from the mains supply.

Had a play with possibly swapping out the main Schottky for a MOSFET, but it's tricky without resorting to a complete 'ideal diode' circuit (three transistors, two resistors). If it bugs me I might just buy an ideal diode module.