I had a little boy climb over my six foot fence last week to get something that had gone over my fence but I don't have a beware of the dog sign on the gate ,so I asked my local RSPCA about it and they said go ask to police and this is what reply I got back .....This will all depend on the circumstances but even if someone is a trespasser, they are still owed a duty of care. This means that if someone was trespassing in your garden and your dog attached them - you could still be liable.
There is an exemption from prosecution for householders where their dog has been dangerously out of control with respect to a trespasser who is in, or entering, their home, whether the owner is present or not. However, this exemption does not apply to dog attacks on trespassers in gardens, driveways or outbuildings. The reason for this is to differentiate between innocently intentioned trespassers, such as a child entering a garden to retrieve a ball, a neighbour leaving a parcel for safe keeping or retrieving garden cuttings, from those with malign intentions. Trespassers in or entering a dwelling are more likely to have malign intent.
Courts may take the view that if you require a 'beware of the dog' sign then the animal is not safe to keep in a domestic setting