Thanks for your advice, makes sense to me. I've redesigned (see below) with a dc-dc dropdown inline (I'm assuming I can feed this to the vin pin rather than to a barrel connector?).
I've got heatsinks ready for my SSRs, but hadn't read about using MOVs so will do (I will, just to be on the safe side, but just for my own education: how vital are they if you're not using a motor?).
The addition of the extra ssr is just so i don't have to swap plugs - in reality, only one of these (plus the arduino and 12v supply) is ever active at one point. The 240v supply will exist in the same project box as the logic circuits, but I'm planning to mount them on entirely separate 'planes' so they only connect at the header pins. Other than these changes, does it look ok - I might start building!