They want me to send my passport as the spouse but it's not in the married name. On my kids birth certs it's my married name too and I think I have to send those in.
The only reason I still retain my maiden name is my passport doesn't expire until 2018 and I refuse to pay 90 before then to change it. But am I going to have to?
No, you aren't.
It's perfectly fine to have the visa in your maiden name and everything else in your married name.
However, do be aware that if you change your name in your passport in 2018, then you will have to pay £223 to change it on your BRP card as well, and you'll need to send off your passport and BRP, which can take a few months. Plus you only have 90 days to change it on your BRP card or you can be fined £1,000 and have your visa cancelled.
So, if there's any chance you might want to change your name on your passport in the next 2.5 years, do it now, before you apply for your FLR(M)... then your visa will be in your married name too.