Insurance legal cover is an excellent point. If you have a strong case, you might find a solicitor willing to act on a 'no win no fee' basis (not the dodgy ambulance chasing kind, some proper legal work is done this way). But, you might not.
One thing you might struggle with, as pp mentioned, is finding a surveyor willing to write a report to be used as evidence against your surveyor. A lot of surveyors won't do this to their colleagues - it's not so much 'closing ranks' as not wanting to be seen as the back-stabber of the local scene. This will depend a bit on how well-regarded and well-connected your surveyor is. You may have to go a bit further afield to find someone (if and when you get to this stage, post-solicitor involvement). Just be careful about the quality. Other local buildings surveyors will probably be willing to discuss who is reputable and who might be worth contacting, even if they won't do it themselves. Also, you must use a qualified buildings surveyor to provide evidence against another qualified buildings surveyor (lots of surveyors only do Homebuyers, BS is a higher professional qualification).
That's all a way down the line though. With luck, your surveyor will be willing to admit to the shortcomings of his report and compensate you. Obviously, what you're asking for is the cost of the roof work, not just a, relatively paltry, refund of the cost of his report.
One point for now. During the evidence-gathering phase, you need to be speaking to people in person and in situ (your house) as much as possible. Looking at the house while talking may prompt observations and connections to be made that would not come to mind otherwise.
Likewise when you talk to the surveyor. Invite him over, walk round, show him the issues, talk, listen and take notes. You need to see and hear how he reacts, as well as listening to his words.
Moving to formal contact by email comes later.