In reply to doc_h:
In theory abseiling outside is in a SPA (single pitch award) remit. However I understand the main insurers for SPA's and MIA's tend to ask for an extra premium for building abseils. The ability to place nuts and cams is not much use when working on a roof! Rigging / bat plates and the ability to estimate structural load bearing would be of more use.
A good professional with a suitable package of skills beyond the SPA / MIA, ie done this in the past would be a better choice than a super keen SPA / MIA who has done lots of releasable abseils from trees / boulders/ nut placements. Being honest, some one with Rope Access skills could do a good job provided they understood client care, problem solving with novice clients and how to fit novices into PPE.
So a hybrid Rope Access, building surveyor / SPA!
Being UKC some one will come along and shoot my message down. But as an MIA, who trains and assesses SPA, CWA & CWLA's I am acutely aware of my limited knowledge of the load bearing of 1960's flat roofs!
Best of luck and I hope you get it all sorted.