So I finally found out that: 'good reason' can be
any reason, as long as it is valid, genuine, plausible and evidenced - evidenced being heavily dependent upon to achieve a positive decision- the discretion of the UKVI/HO whatever you want to call them, have to be directed to use their discretion and actually consider the evidence.
not hard to understand - but the lack of reply here and the manner of previously replies is toxic- and uncalled for...
being 1 day late on submission for ILR - isn't as shocking as one may think, options are always open, especially for those with families and ties to the host country (at least considered by the UK).
I am currently waiting on word for ILR if that's not granted I'll likely get a 10yr route - at least UKVI/HO will be directed that way if they reject the 'good reason; if the 10yr route is granted I'll be able to switch back to the 5 yr route (under the 14 days rule), and apply for ILR (ok I'll have paid twice- a small price to pay noting the initial submission was 15hours late - at no fault of our own).. if all else fails.. the fact we have children, both British Citizens, at school and as a family unit have ties to the UK - cannot be ignored by the UKVI/HO - which would actually be a guaranteed issuance of the required visa... however noting we cannot go directly to this process without first exhausting all others ..
it also helps I took professional advice from a leading immigration firm of London Barristers! I suggest anyone that is in a complex situation at no fault of their own - and have the available financial resource at hand- get a Barrister!! I've seen that written elsewhere and only ever took it as overkill - but trust me- it's a weight off our shoulders and mind!!
Finally, if you can avoid for whatever reason 'overstaying' avoid it at all costs!!!! more-so of the sleepless nights... but to understand there's always help, even if it has to be paid for, is still reassuring...
a moot (positive) point - at least I'm off work till I get a decision on my visa/status

)))