This point seems to crop up much too regular for my liking, I think if someone could clarify once and for all, it will be a great help.
I too thought Visa expiry for Immigration purposes was as stated on the actual visa, but having spoken to IND when Elsa applied for her ILR, it was confirmed the visa expires on the anniversary of arrival in the UK.
Now, I'm not sure if you can apply this to Fiance Visas, but I can say for sure, the arrival date rule applies for Spouse Visas, and judging by other threads:
http://filipinaroses.com/showthread.php?t=2511
http://filipinaroses.com/showthread.php?t=1086
I don't see why the "arrival date" rule does not apply to Fiance Visas as well.