OK, for what it's worth, I have checked this out with my local Sure Start office this morning.
This is their response, cut & paste:
"You must allow parents to access just their paid entitlement, at present 2 x 2 and a half hours =5 hours in a day. So parents must be able to just have the early education sessions in 1 day. They may opt to pay an additional hour and stay for 6 hours, firstly only if it is offered , secondly then only if they wish to purchase it.
Prior to September 2008 when EYFS was introduced groups used to be registered as either sessional or full daycare groups. Where a sessional registration was held there had to be a break if 2 sessions were offered in 1 day. Now all preschools and nurseries are registered as "childcare on non-domestic premises" and this does not refer to the hours or sessions.
Some parents if collecting other children from school would prefer to not have to trek up to school twice, so would prefer to purchase the additional hour. If you have children attending the morning sessions only, you can offer an additional 1 hour lunch club at a chargeable rate, hence the title flexible offer, offering flexibility of choice to the families you serve".
[misshardbroom goes to lie in a darkened room and BREATHE....]