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?).
Not Vin - see the schematic:http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
You want to feed regulated (it -must- be well regulated!) 5 VDC power to the +5V pin, which bypasses all on-board regulation of the Arduino; Vin still goes thru the on-board voltage regulator - it is basically meant to be used when you don't want to use the barrel jack (or USB power), but you want to still use the on-board regulator.
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?).
Here's a pretty good document on SSRs and details:http://literature.rockwellautomation.com/idc/groups/literature/documents/at/700-at001_-en-e.pdf
They are mainly meant for inductive loads - so if you are controlling anything like a motor or a transformer, you'll probably want one. But - you may want to check the datasheet for your SSR first - some SSRs have such "snubber" circuitry built-in (and some use a much better device called a "transorb")...
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!
As a block diagram it looks ok to me; as long as you make it safe from shorts or other issues between the high and low voltage sections, it should be alright...