Pedwardine, I don't think you need to keep the incomers seperate for 6 days unless you are wanting to move stock off your own holding within those 6 days. If that were the case, then the incomers would need to be in a Defra-approved isolation facility in order to not trigger a holding-wide 6-day standstill.
But if you don't need to move anything off your holding for a week after the incomers arrive, then the 6-day standstill won't cause you any inconvenience, so you wouldn't need to isolate them - unless you wanted to for your own reasons.
Re: infectious abortion, there are several causes but the one I was thinking about is Enzootic Infectious Abortion (EAE), caused by chlamydia. It is spread by contact with aborted materials from infected ewes and by contact with vaginal discharges from infected ewes at tupping time.
Infected ewes will usually abort their next crop of lambs, or have weak lambs that die shortly after birth. Thereafter that ewe will be immune.
You clearly don't have EAE in your flock at the moment, so it would probably be prudent to require incoming ewes to be vaccinated in order to ensure that they can't be bringing it onto your holding. They could become infected by contact with infected materials / discharges on their home holding, or such materials being brought onto their home holding by wildlife at lambing time (from infected flocks on neighbouring holdings.)
Alternatively, as you are ceasing to operate a closed flock, maybe your best option would be to commence vaccination against EAE in your own flock, then you can be confident that your own sheep are covered and cannot become infected, either by incoming sheep on tupping holidays or by infected materials from holdings neighbouring yours. (Which vaccination would also give the owners of the visiting sheep security that their ewes wouldn't be picking up the infection while on your holding.)
Your proposed charges sound eminently reasonable to me.