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If there are not currently any leaks them personally I wouldn't bother. Surveys never ever come back with a clean bill of health.
If he/she removes all the moss (who gives a fuck about that, it'll grow back anyway) and fixes the small pointing jobs, then I'd expect 1/2 - 1 day's work, so £200 - £300.
If you want evidence of the issues (and latterly the repair) then ask for photographs to be taken.
My understanding of what he needs to do is:
This all seems straight forward and uncomplicated so there shouldn't be any risk of him fucking it up - other than possibly breaking tiles.
The obvious place for him to scam us/add extra seem to me to be:
I realise this all sounds a bit neg. But he's annoyed Mrs. Hugo7 and I'm now being quizzed on what he's actually going to do and how we know it's been done.