Any experience of dropping back a year to start primary?

[Miloarmadillo1]

My DD (2y4m) has West syndrome and resulting GDD, plus growth issues and we are currently investigating if she has a genetic disorder tying that little lot together. She is actually doing really well developmentally and has significantly narrowed the gap with peers, so at the moment she's only mildly delayed. It's likely that she will have some residual issues affecting her learning (autism, ADHD, language delay, processing disorders, dyspraxia all common sequelae) but we don't know what or how severe as yet.
Since she is a summer baby she doesn't have to start school at 4, we can delay a year. We'd only do this if LA will let her drop back and start in Reception. I can see what a big difference being the youngest has made to DS2 compared to DS1 who was almost 5 when he started, and that was without any SEN to consider.
I'd appreciate any insight from anyone who has done this. We have a year to think about it since we have to apply next year even if we want to defer until 2019.

[Dustpan]

Watching this thread as I'm also interested in whether I may need/choose to hold DS back a year. He is 2.3 and a summer baby. Has a speech problem that looks like it could be severe, plus some sensory/ attention hyperactive issues for which we've also been referred. If holding him back a year means his speech is stronger before he starts school, I think that might be an invaluable foundation for the rest of his school years.