Ok, so in 2014 I married my husband and at the time I applied for my second visa which was the FLR (M) what I did not know at the time was that it does not actually last you for 5 years, which is the route I took, but they split that up into 2.5 years and after 30 months you have to reapply. So my Visa expired, TOTAL mistake by my part. It has been the biggest mistake I allowed to happen. Anyway, in that time I had our son who is now 2, bought a house together etc... So after a huge mess of complications I have hired lawyers to help me through the issues.
How long ago did your visa expire?
If it was within the last 14 days, you may be okay, but you would need to get your FLR(M) application in before the 14 days are up. However, you would need a very good reason why you could not apply before your visa expired - such as emergency hospital treatment or a family bereavement.
If it was more than 14 days ago, the safest thing to do might be to return to the US ASAP (you don't want to overstay any longer than you already have) and apply for a spousal visa from there, rather than trying to apply 'outside the rules' from within the UK.
See here for information about the 14-day grace period:
https://www.freemovement.org.uk/overstaying-applying-leave/For better advice though, I would recommend contacting Medivisas, an immigration firm based in London who specialise in family-based visa applications.
I was advised to fill out the FLR (FP) form by the home office based on information I gave, but now the lawyers are saying that I (may) need to go down the FLR (M) route.
FLR(FP) is for people who cannot meet the financial requirement for the visa - it's a special circumstances visa which will put you on the 10-year path to ILR...which means another 10 years of 2.5-year FLR visas.
I'm not sure if this would be the right visa in your situation or not though.