Members of the Institute of Structural Engineers are the people to call when work has to comply with building regulations, but doesn't justify the services of an architect. Structural engineers will also undertake consultation work. For example, when you knock two rooms into one, a structural engineer will specify the type and size of beam that has to go over the new opening. If you are using an architect, they will consult a structural engineer as necessary.
What Does it Cost?
There is no fixed scale of fees - charges are negotiated according to the work involved and vary from one part of the country to another.