Lots of questions, and you'll think of lots more
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You'll want to get your herd no. now, as whilst legally you can do this within 30 days of getting your pigs, you'll need to register with the electronic pig movement system called e-aml2 - website
www.eaml2.org.uk/ - all pig movements are electronic. So go through to your local animal health office and get a herd no.
An 8*6 ark will be fine, and allow you to expand your plans without needing to buy a new one. However when you get your weaners home, you might want to put a couple of bales of straw in (uncut) to reduce the inside area down a bit - otherwise your weaners will leave their bed in the corner of the ark, and pee and poo still in the ark, as they mentally have left the sleeping area. Reducing the floor area when they are very small helps them go outside to do their daily !