How to get dry rot sorted

Phil42_2

We have a Victorian house. Just seen evidence of dry rot fungus seeping from skirting board in one small corner of front room which we don't use very much. The other side of the wall got wet some time ago when a downpipe was leaking. It's also above a damp basement.

I am aware of the potentially scary nature of this so you don't need to convince me that it's not sufficient to just get the small visible evidence, skirting board and plaster dealt with. I understand the need to try and get to the underlying causes.

The question: what's the best way of getting this solved without being ripped-off?