OldBloke

"Can anyone recommend a good Universal power supply."

The APC Back-UPS ES series you mentioned is very good.

"but don't know if that too much voltage for my needs - they also do a 400va version too."

They all have the same voltage, 230v in the UK. What matters is the va number, which is about the capacity of the battery. The 400va model won't run the computer for as long as the 700va model.

I got the 700va model. It tells me it can operate the system for 37 minutes if there is a mains powercut.

Looking at your specs, it seems we have very similar hardware, except you have two monitors and more hard disks.
If those are LCD monitors, not CRT, I guess a 400va model should be sufficient. It will give you enough time (maybe 15 minutes at a rough guess) to save your work in currently open apps, which is the main thing.