Thanks for all your messages. It's really good to hear your opinions.
My current intention is to keep them together but I have requested that the school sit them at different tables and encourage them (not force them) to socialise separately. I know they do a lot of group work and will spend time doing different things - particularly because my DD seems quite academic and my DS is more interested in charging around. I'm sure that is normal for their age but they have been at nursery since 11 months so we've got a good record of their progress/preferences.
I have asked the twins what they want to do on many occasions and my DS (who is slightly more insecure) wants to say with his sister and my DD (more confident) isn't fussed.
They will have to 'go up' to the Juniors school in Year 3 so perhaps then might be another opportunity to re-evaluate the situation..?
From a selfish point of view, I'm a single mum, working full time and commuting 3 hours each day to make things work. Even with a co-operative ExH and a lovely XMIL, supportive parents and wrap around child care, our schedule is very tight. Swimming lessons are squeezed in on one evening and they spend every Fri/Sat with their dad. Separating them into separate classes just might open us up to more chaos with school trips on different days, parents evenings at different times etc.
Furthermore, I certainly didn't expect to be divorced from their father. Although we both work exceptionally hard to ensure they feel secure, I think the feeling of guilt is a hard emotion to bury.
JG