Thanks for the string-gauge advice, I'll go for Regular Slinky. But it was the length of the slender E which surprised me - seriously inadequate.
My new steelwork inside isn't pretty, but it's unseen and it's rigid and I like that, better than springs. It's just a couple of 4-inch M5 bolts through bits of plate.
I didn't much admire the dirty screws and half-painted corroded claw that I found holding the springs, either - not one of Leo's finest.