My Fiancee Visa was set to expire 6 March, and we set the wedding for 21 March because it was the first day of spring and we wanted our wedding date to mean SOMETHING to us - not just be dictated by the 6-month time limit. (We weren't even really engaged when I came over - but that's a whole other story!!)
Anyway - about a week before it expired, we went down to the Home Office with all of our proof that the marriage was indeed scheduled and we'd paid our fees to the Registry Office and had our names listed for the requisite time and so on and so forth. Told them it was the soonest we could get the relatives I wanted to attend over from the US - and they gave us two-month extension. No problem really.
The form you'd fill out is FLR(O) (Further Leave to Remain - Other). The Home Office website has the form you can download, and it lists very explicitly everything you need to take with you in addition to the bits about the wedding bookings and such.
Good luck!
