Most surveyors will admit they are not qualified to give an assessment of the condition of a thatched roof.
Don't put an offer in on a property unless first at the least you have spoken to a thatcher about the condition of a thatched roof and preferably after you have had a survey carried out.
Don't take it for granted that what the thatch owner says about what work has been done on the roof is entirely accurate - some thatch owners get confused between re-ridging work and repairs and re-thatching.
Do get a written survey from a thatcher.
Do ensure that the survey gives prices at todays prices for what work will be needed even if it's not for 10 years from now.
Do have a good look at the roof yourself - if something looks untidy question why this is.
Do ask the thatchowner for the details of the last thatcher to work on the roof.