Pruners, I haven't read all the responses but my DS1 has a January birthday and we delayed his School entry by a year. This was partly on the advice of an education professional (sorry I can't remember what the woman's proper title was it was a while ago but something to do with assessing his ability for playgroup/nursery to determine whether he needed extra support etc), DS1 had a speech delay and this woman advised keeping him back and year and said that if we needed a letter she would happily provide one, even although DS1 suddenly came on leaps and bounds and was no longer considered speech delayed by the time he took up his pre-school nursery place.
However DH and I would probably have tried to defer him anyway and had deliberately no put him into a 'funded' preschool place until he was 3 1/2 so as to avoid any issues with him receiving more 'funded' preschool education than he was entitled to.
To cut a long story short-ish DS1 attends a large Primary School with two intakes every year, they tend to divide the classes with the older ones in one class and the younger ones in the other, DS1 ended up being one of 6 who had been deferred.
I personally think it has been a good thing for DS1. I have a January birthday too and went to School at 4 1/2 (don't think deferring was so much the done thing back then) and sometimes think I would have benefitted from being a little older when starting School.