Ok here's an update on progress.
I've confirmed the 20A MCU fuse has blown, no surprise there, and I can also confirm that one of the two blue caps has split, although not as violently as some other cars as there is no debris anywhere, just a clean double split of the cap:
The resistor I was also a little bit worried about at first as there is a black mark on the corner near the capacitor however on closer inspection I think it is residual potting mix or perhaps just the effects of heat build up over many years heating the potting mix causing it to wick up the side of the resistor:
It doesn't look like an actual burn mark on the resistor and there is no melting of the potting mix at the base of the resistor or the cap itself. So I will measure the resistors from the top and verify relay operation - if they both seem OK I'm not going to replace the resistors because that would require waffle plate desoldering which I am not currently equipped to do.
I presume it's not possible to do meaningful diode checks on the waffle board without desoldering it ? If so I'll just have to take the chance that there is nothing blown on it.
I've decided not to remove the whole OBC box from the car - the outsides are very corroded as are all the mounting bolts, and removal would require work outside the car (not possible with current weather) including draining and refilling the coolant - and I don't have any to hand. (And funny story, my other car, a petrol car broke down terminally last night so I have no working car at the moment making this OBC repair super urgent...
So the plan is to remove the bottom board from the box together with attached waffle plate to take inside for repair as it looks a bit too awkward to work on the board in the recess of the box hunched over inside the car to stay out of the rain - especially the potting mix removal.
I've also checked white fuse F701 on the underneath of the top board that was reported as faulty earlier in the thread - it measures ok.