WB,
agree with HM - keep the sows in small groups - we often bring sows onto the yard in pairs/3's two weeks before farrowing (to increase feed) and then split off just prior to farrowing.
Legally there are several factors with creating an isolation pen for defra approval (to allow you to bring pigs in and not be on standstill) , email oaklandspigs@btinternet.com and I'll mail accross the reqs and form the vet needs to complete early next week when I can track them down on the other PC. However as a rough guide, if in a field you need to achieve a 3m seperation and have this approved by your vet who then sends to defra for their approval - typically takes a couple of weeks. Two rows of stock wire 3m apart will achieve this, just need to consider where wee and poo will drain to, so not on steep slope. If building, then more needs to be thought about - but think you are just in fields? You need to have foot disinfectant dip, and seperate overalls/wellies and we recommend feed last. We achieve this by having a dustbin with food by the isolation and bucket/or scoop, together with a second dustbin holding overalls/wellies (so they don't get wet in the rain!). On feeding we dip existing wellies, take off, put overalls and other wellies on, feed, dip wellies on exit and remove + overalls, re-store in dustbin, put old wellies on and re-dip. Bit of a regmarole, but quick and easy with practice and afterall you want to ensure you keep any germs in the isolation.
You need to be able to unload directly into the isolation.
As HM says, greater distance is better, but legally it's 3m !
If you are not worried about standstills, so don't need a "legal" isolation, then above will give you a good guide to think about.