we found ourselves in a similar scenario in august when up in the lakes, where sheep have a tendancy to suddenly pop out from the bracken with no warning. barkley just went loopy and my oh had one particular nightmare walk when barkley spotted a sheep before he did, and was gone after it before my oh could react. we don't think he harmed the sheep but he definitely now carries the label of 'sheep worrier' and we are anxious to try to sort it out. the long line idea is a really good one and we'll have to try it.
in fairness to barkley, he is getting better all the time. one of our walks takes us past fields of cows that are easily accessible. i'm now able to stop him from entering and after a few quivering moments stood looking in wonder at these big mysterious creatures, he'll drag himself away and continue on his walk. sheep are another story because they run away.
hope you manage to come up with a successful strategy that allows you all to relax and enjoy your walks. the new forest is a wonderful place.