Hi Tracy,
there\'s 2 issues here, one is the right or otherwise of people to enter your land, the other is dangerous dogs (whether they are on your land or not).
In my experience there is a difference between loud/agressive and dangerous dogs, but that doesn\'t necessarily make the experience less frightening if you meet either type. So depending on the particular dogs (and owners) in question there are various options. Step 1 should always be to talk with the owners and explain the situation (e.g. that you are OK with people walking dogs in the wood, but that the dogs should be kept on a lead due to the wildlife, etc) - this is a better 1st step than criticising someone or their dog as people can be sensitive even if they appear otherwise. If talking doesn\'t work, then putting signs up probably wont work either and you probably dont want to physically prevent access to all your woodland; if it does come to it that the law needs to be involved, then I doubt that a PC will be interested unless you have proof that a criminal offence has taken place (trespass is not a criminal offence), so unless you personally know a copper who you can ask to do you a favour you may have trouble going down that route.
However, if by \'very agressive\' your friend means the dogs were \'dangerously out of control\', then it is a criminal offence and plod may be more interested (especially if you have evidence).
a quick search found a couple of handy website summaries;
trespass summary http://www.a-level-law.com/tort/trespass_to_land/notes.htm
dangerous dogs and law http://www.defra.gov.uk/animalh/welfare/domestic/dogs.htm
Binz
[don\'t judge a dog by it\'s owner, or an owner by it\'s dog]