Ther3 really shouldn't be an issue with 230v and 240v. The UK is nominally 230V now, but unless you are supplied by a recently replaced substation, we'll still get 240v or more, not 230v. And CE marked equipment (like the Marshall should be) is designed to work on 230v +10%/-6%. So should work OK from between 216v up to 253v.

680V sounds weird, as 450vDC at 230v should give a maximum of 469v DC at 240V. The main smoothing caps are rated at 500VDC, so they would have all blown up at 680v. 680v sounds more like the peak AC voltage you'd get before any rectification and smoothing. The schematics I found on line didn't give any test voltages, which isn't very helpful.