Um! This will usually be done by the boat owner.
When making the decision about buying a boat, the owners practical skills should also be taken into account!
Carpentry work, Mechanics, Engine services, Electrics, Boatmansship, Map reading are some of the skills one will need in the ownership of a boat. To attempt to own a boat without at least a small knowledge of any of theese skills is, to my mind,shear folly.
I would say the chances of getting hold of a Carpenter/ Plumber, whatever he is, are nearly imposible within the inner bounderies of the M25.
You may get someone who says he or she can do it, but i doubt if he will be a qualified Carpenter or Plumber.
In any event be prepared with a very full wallet should you find one!
My suggestion is to get hold of some tool and some time and do it yourself, that way you will learn a bit at the same time!
Sorry to be so negative but thats the way it is!
Nipper