Not daft at all, slices! HG's response seems pretty thorough. My hubby & I ran into problems with this when I was moving over here to England, and we didn't have as much info to work with as HG has provided.
We applied for the fiancee visa mid-Feb in 2004 & I guess we somehow assumed they would do it based on the date we filled in the 'when will you be entering the UK' question (something to that effect). I was not moving over until March 28. Since it seemed we had a 6-month window for the marriage to take place, my DH made initial preparations on this end by reserving a Sept 25 date in the Harrogate register office. Of course, all of this ended up being a right palaver! We had selected the Sept 25 date as it had some correlations to both of our parents' weddings and so it was meant to have some additional personal & sentimental significance, etc. When I received the visa, it was -- of course -- immediately dated for Feb 19th! Which meant it expired on Aug 19th -- about a month & a week before our wedding was scheduled!
We went to see an immigration advisor after I got over here to see what our chances would be of having the visa extended so that we could stick to our originally scheduled wedding date. The advisor basically told us that it's just not done, so that completely discouraged our original plans. Phooey! By that time, there were NO decent marriage ceremony appointments available at the Harrogate register office (only times were weird weekday times). We live in Leeds Council area so I called our local office instead - same thing! I began calling around at licenced, approved civil ceremony venues...eventually we lucked out! A lovely small local hotel (country house style) had just had a wedding party/ceremony cancellation for Saturday, August 14th! We grabbed the date -- couldn't believe our luck because it was like July 1st by that time!
We posted the FLR app in on the Monday right after the wedding -- 3 days before expiration! It took them about a month as I recall to process it & return the FLR to us. Of course, we had to postpone making any honeymoon plans until we received the FLR (and our passports) back from them. Had a lovely trip to Spain afterwards, in October.