According to a skylight contractor who came yesterday the roof is moving apart (as a result possibley of the shed extension removing a bunch of equalateral rafters along one side) and from the garden one can see slight bowing along the legth of the gutters. He reckons the roof needs one or more "ties" to hold the walls together. He reckoned its no big deal although the aesthetics of covering up the "ties" might be a challenge.

Any thoughts on this - problems, cost etc? What sort of contractor would one contact?

Pitched roofs produce a spreading force on the structure.
The ridge tries to move down and the rafters push outwards ion the walls.
The ceiling joists (attic floor) are in tension and resist this spreading force.
Collar ties can also be used, but they increase the size of the rafters required.
A structural engineer is probably required at this point to figure out what is going on and try and design a fix.

sheshe