We paid £600 to get the house surveyed before we bought it....

[TheNoiseHurts]

...just in case something huge like the roof needed doing.

We got what we thought was a decent surveyor with good reviews.

We have lived here 6 weeks, the roof is leaking and is going to cost us 8-9k to repair.

What exactly is the point of surveyors?!

Fuck.

Well yes, mentioned as a discouraging qualifier in the very post you've quoted from.

(I've been in a situation where the seller did answer a pertinent question untruthfully on the conveyancing form and I could have taken action against them).

On the question of what you'd get if 100% successful... you cannot know that until the strength of any case you might have is assessed. You could end up with all the necessary work and all your costs being covered by a payout from the surveyor's insurer. Or not. Or something in between.

If it got as far as a legal case, what you'd be hoping for is an out of court settlement. This is arrived at through a process, effectively, of betting against the likelihood of being 'found against' in court. The more likely your surveyor's insurer thinks that outcome is, the better the payout, based on your evidenced claim, you'll get.

I agree the best case scenario is likely to be an out of court settlement

I think a previous poster is right, the loss would be diminution in value rather than the cost of repairs. The difference between the value of the house with the knowledge of the roof problems and the value of the house without the roof problem.