Thanks for the feedback Robin. Yes the calorifier is about 80 litres, but playing with the calculator, having the pump at 2,5 Bar, the PRV at 3,0 Bar and the EV pre-charge pressure at 2,3 Bar is a crap combination. Depending on the temp range it comes out with an EV of 50 litres plus.
If I can get the pump cut-off down to 2,0 Bar as Pete suggests and keep the water temp range 10 - 85 Deg C it comes out where you're at. I think the nearest standard size EV is 18 litres, still physically challenging to find somewhere to fit it!
Apparently the FloJet is adjustable so it's beginning to sound do-able. Cheers.
Edited for crap syntax.