Could someone please advise me if mating a wire dog changes its nature. I have heard that mating can cause behaviour problems i.e. agression. I have been asked by a breeder to allow her to use my dog but am not sure how to go about it.

Yes it can change the dogs behaviour, it can become difficult to keep in, will become whiney if they smell a bitch when out, they can become dirty marking within the house and wary of other males who they will see as opponents. If the 'breeder is experianced then they will know what to do and the sort of health tests that you will need to do, but you will be responsible for any puppies born for the rest of their lives. The fee, about the cost of one pup, so unless you intend to continue to use your boy at stud, is it worth it?