Thanks all for the advice.
By way of an update, we took the sow out of the boar pen after about a day as neither of them seemed particularly happy with each other.
She then came into season and we took her to him, keeping her outside of the pen at first. The first time she still seemed quite aggressive towards him so we put her back with her sisters but on the second visit a couple of days later she seemed more interested and less aggressive. We put her in with the boar then and there was a bit of scuffling over food but nowhere near like the last time.
She has been in for a day now and they seem to be getting on much better, even going into the ark together whereas before the boar was hiding from her. Fingers crossed he'll woo her now!
We are just wondering now whether we should take her out when she is no longer in season? Is she likely to become aggressive again or are we past that?
We had planned to leave her in for a couple of months with him, then rotate in one of the other sows when they are ready.
Many thanks for your help,
Zoe