Yes, you are allowed to have extra children in an emergency but, as you say Alison, that "emergency" isn't supposed to last for the rest of the day. Technically, if you collected a sick 3yr old from nursery, which meant that you had too many under-fives, you are supposed to make every effort to reduce the number back to your legal level asap. If the unwell child can't be collected for several hours (perhaps parents went to Isle of Wight on business for the day??!), you should send another child home!!! In reality, I don't think any of us would do this!
And another gripe - if you have an under-five of your own, they will be included in your numbers even if they are not on the premises. If you were asked to have one of your mindees for an extra day, for instance, and sent your child to grandma or paid another minder to have them for the day, YOU are still seen as being responsible for them. You cannot "give" that responsibility to anyone else.
OFSTED have been known to tell minders that putting their child's own daddy down as emergency contact for their nursery (so that you could have 3 under-fives to mind) is also not always acceptable.
By the way, I don't make these things up as I go along, these are "incidents" that have cropped up during OFSTED inspections with other minders.