Visa Timeline: Length of Stay - Time Needed Away

Hello, All.
I'm having trouble deciphering all of the info on the web pages I've found so far - including UKVisas and the Home Office pages. Hope you can help...

I am an American from California who entered the UK on September 1, 2006 and was issued a six month Visa at the airport. I will have to leave in March 2007. A few different questions based on the following scenario:

* If I leave the UK for the entire month of November and comeback Dec 1, does that mean my Visa would be extended to April 2007?

* Because I only stayed in the UK for 2 months (September/October), can I apply for another six month visa from the Los Angeles based UK embassy when I get back to California or will they only grant me the remainder of the time which is four months?

* Realistically, how much time must I wait in my home country (US) in order to be granted another another six month visa?

A side note is that my girlfriend (UK citizen born and bred) and I plan to get married, but don't want to HAVE TO rush the dates in order to conform to the government's timetable just so we won't be separated for too long, but we're finding we may have to rush so we don't get stranded a half a world away from each other. We desperately want to do this the right way...

Many thanks in advance for any help or direction you can provide.