Yep I just realised that the 4000 on the input ranges actually refers to 4,000k
and 1,000v max input DC and 700v AC so it's not a major problem after all with a little thought, I can use 230Vac as the input source for both the 400v and the 750Vac ranges and it will be near enough for function. When it comes to DC, I have my Heathkit RC bridge which generates around 360Vdc and by comparing the readings against a known good meter, I can then enter those values into memory and it will fit for my purposes.
I'm going to hold off doing any more calibration on this until the correct battery arrives, hopefully tomorrow, then I can reassemble it correctly with all the proper screening in place and then it should be job done, another British built piece of TE on my bench