Based on my understanding I was convinced that my wife's settlement visa expired 30 months after she arrived in the UK. That would mean applying for the FLR before the 8 Sept.

Only after seeing an Immigration solicitor did the solicitor point out that the visa expires on the 18 Nov. I don't want to risk my wife travelling and not being able to return after the 8 Sept but before the 18 Nov.

Was my assumption wrong?

I even went as far as asking the Immigration officers at Heathrow, they seemed fine with the 18 Nov but as they said they are not UKBA, I didn't realize that and said I should talk to UKBA.

Her initial visa was valid for 33 months, so will expire 33 months from the date it is valid from, which from what you say is 18/11/15, and can be used to enter the UK any time up to and including that date.

However, the residential qualification for FLR is 30 months, so she can apply for FLR any time between completing this and the day her visa expires.

Actually, she can apply for FLR up to 28 days before the end of that 30 months.

"Actually, she can apply for FLR up to 28 days before the end of that 30 months."

I think that was caused my confusion, I know it was from Aug I could apply but didn't realize it could be left right up until November.

I think that works better as I assume that the FLR is 30 months from date of issue. Based on my initial thoughts that would have put the ILR in March 2018 so the P60 would have been almost a year old. As it is the ILR will now move to April 2018 so I will have an up to date P60 when I apply for the ILR.