Among other things U need to determine if the volunteers can be seen as true stand alone volunteers [people] doing their own thing and using U as the vehicle to do it e.g. a parent going into a school under their own steam to paint a classroom over a holiday break who is not part of a formal charity or part of the formal set up where they could be deemed to be employees so/or are seen as part of the charity as the latter needs looking at in dept and the former should relieve U of most if not all problems noting that there is nothing in the HSE guidance for the former situation and in theory S:3 can apply but once U get into a foreign country U must abide by that countries laws etc. as the primary duty

Read as much H&S case law, news reports and similar as U can on the subject and get your insurers involved as I believe that there is employment and tax case law that takes the stance that volunteers under certain situations, as the latter above indicates, can be seen as employees. Also get in touch with your legal specialists as this is a very complicated area and contact other charities who are doing the same thing to c what they are doing