Assuming you have the permission of the leaseholder to make these changes, then you could try Roy Darville who has done some work at a couple of properties at RQ. His email is email@example.com. As is common with a lot of plumbers he is often quite busy so he may not be able to react in a hurry.
Many people have used Ian Norton over the years. He's not the cheapest by any means but all reports are that he does a very good job. His email is firstname.lastname@example.org and he knows the site well.