visa's until recently use to be valid for only 24months and basically you had to be in the UK 23 months (so you had to leave within 28 days of the issue date of the visa) if you left any later you would probably have to apply for FLR (further leave to remain) to extend your visa til you had been in the UK 23months and then you could apply for ILR.

you really don't want the hassle of applying for another visa and paying the fee unless you really have to (my wife had to, as she had a 24month visa and had to take exams in the phils so she had to apply for FLRin the UK before her spouse visa expired)