Oh wow so even if the visa say it expires in April I can apply before then but still have the 8 weeks open and I can still work and use the NHS during the 8 week period without it interrupting my work schedule ? This is really good information as always I will consider the 8 week processing times if nothing is correct with in the next 2 months
Yep - the only thing you have to do is submit the online application before your visa expires... you don't even have to have attended biometrics yet, because your official application date is the date the online application is submitted.
As long as you make an 'in-time' application (submit it before the visa expires), your spousal visa and right to work will be automatically extended under
3C Leave while the visa is processing.
It sounds like the free biometrics appointments can have a wait of a few weeks or even a couple of months, so you could schedule your biometrics for a later date, and then hope it takes a while for the visa to be processed after that (although there is always the chance that it might be processed in, say 4 weeks, and not 8 weeks).
BUT as I said, this is not guaranteed... you could do all of that and still find that your FLR(M) expires before you can qualify to apply for ILR, which would mean applying for another FLR(M) to bridge the gap.
You have until April to get the correction sorted out though, so hopefully you won't have to worry about applying in April, and can just apply later in the year.