OK so today I cut the Valchromat to almost finished size, it's currently 1205mm wide but I'm leaving it for the time being until I've had a full proper test fit, I can always skim it off with the track saw.
The mitre seems OK on a rough test.
The front panel has also been cut to size, I'm not sure on which method I should use to fit it all together quite yet. Glueing the two valchomat panels together is one, which means they'll be joined at 90 degrees independently of the base. Downside is that I know I won't get it exactly on 90 degrees so is there any point?
Second method would be to put the side panel on first, because then I can trim it and match up the top panel which is easier to do if the vertical one is glued in place. Only problem with that is that if the join isn't 90 degrees there's no way to adjust it.
Third is the reverse, so the top panel first, and then put the bench on its side and fit the vertical one. The benefit with this is that the top has a lot of planes where it sits on the frame, and if I glue it on first, I can tinker with the side panel (only a quarter of it touches the frame) to get it exactly 90 degrees. This seems to be my preferred option right now.
Mulling over what to do about the MFT aspect, I think I'll just scrap my 100mm spacing idea and use the Parf guide system despite the cost, because it gives me the comfort of doing it right. Probably.