JSwan, you MUST go. I have friends that live in Somerset. I stayed with my friends and hunted nearly every day for 2 weeks. I've never been more wet nor had a better time out hunting.
Went out with the Staghounds once too. Exhilarating!
Dawglover, sounds like you had the real Exmoor experience - proper job
Whicker, since its you I will reveal what little I know Personally, I've got a Barbour breathable coat - not waxed, more like a Gore-tex - that I love - never, ever got wet in it and it doesn't rustle. However, lots of people swear by Ri-Dry, and their coats are more in a hunting style - website is here: http://www.ri-dryclothing.com/
The biggest problem IMO is waterproof trousers - the Ri-dry ones don't really deliver, and the ones which are really waterproof don't come in the right colours(!). I can give you a steer on waterproof gloves though (ESSENTIAL!!!!!!) as Sealskinz and Seeland both do great ones
As far as tack goes, I don't think there is a way to waterproof it...The only answer is clean it meticulously - strip bridles down every time - and when tack comes home sopping wet I bring everything into the house to dry and clean it with Effax cream soap as it dries. Seems to work
Alternatively (but personally I wouldn't make a habit of it) there is nothing like getting really cold and wet to make you appreciate being warm and dry again after hunting