Hi muddling,
We started DS2 (major language delays+sensory issues) in reception at 5.0 and there is a consensus that this has transformed his life chances. He needed the extra nursery year and he needs his peers to be his age and younger rather than his age and older. His education will be "offset" (ie he will stay with the same peers) until he is 18. We are in Bradford LEA. I recommend the use of the word "offset" by the way- people find it harder to use against you than "delay" or "deferral" and it makes your expectation that this is a permanent choice clear.
If Ellenjane's statement that "The law on 30 DC per infant class means that in a fully subscribed school, they have to give the 30 places to children who are the correct age before they can look at those like your DS. He cannot, legally, take a place from a child who is the correct age who wants a place" is based on a law coming into force Sept 2010 or later then go with that. But otherwise check this carefully - maybe EllenJane can expand? My DS2's application was assessed for his deferred year exactly as it would have been a year before - by sibling status and distance from the school. Be careful, very careful, of words like "law" and "legal" generally. Policies have a different status to statutes but may be described as "law" by those who oppose you.
As I understand it, you have the support of various professionals (EP, Inco, SENCO) for your position. This is absolutely fantastic and is your key starting point. Can you get this recommendation in writing as soon as possible? I'd start by writing confirming what they recommended and confirming that you agree with their view that this would be in DS's best interests. You need to reinforce to outsiders that you and the professionals have a joint agreed position.
As soon as you have done that, you've got something in writing to show the bureaucrats in your LEA when they start to try to block you.
Next, when your professionals have confirmed their position in writing (so can't deny what they said) have a think about which of the professionals has most clout and ask them off the record whether they anticipate any trouble from the LEA based on their experience with other children. If you possibly can, keep them as allies, think of them as key players on your team that you have to coach to get you the result you want.
Of course, you may have to get very antagonistic with the LEA but fight smart.
I have posted on many threads on this topic, usually in education, you may be interested in digging out a few.