Few days old but...had a job a few weeks ago,lady away,daughter looking after house,32A MCB goes,fish die,freezer down,blah blah. Reset,lasts a day,resets again,another week,trips one night,wont let her reset,sparks etc (her words),morning after,resets ok. Only RCD is on stand alone DB shower/cooker,ALL other sockets off this one MCB,a proper hotch-potch of additions,part ring,part spurs and add-ons,recently added kitchen circuits,conservatory,garden lights etc...a real pain. IR at board similar to OP's,splitting as could only "splitting the difference" and not revealing any specific areas...getting ready for the big dissection and thought how hot the weather had been,appeared to be blowing at end of day when cooling,had a wild notion maybe conservatory getting biggest temperature change...so 4 double SSO in there,pull faces off,third one has big plasma shadow,end of 3.5mm screw rounded and melted,small burn on looped line. Big amount of luck,but i'll take that. So,long winded way of saying,a big overload,can be intermittent,without showing up between faults,as a catastrophic IR fail. In other words,you could still have a fair problem to find. Good luck.
