update: my wife is in a government scheme. we've found a telephone number for the central government admin. (66) 029562722. from this you can ask for the number of the particular team that administers the employer who has paid the insurance contributions. it appears that if the Thai national continues making payments until they are 55, they will get a monthly pension. if they stop the payments, they will no longer be covered for health costs but they will get a one off lump sum at 55. figures will vary according to contributions made over what time scale.
it would also appear that in order to cancel the payments, the Thai national has to present themselves in person to the relevant office in Thailand, although we're still seeking clarification on this point - something to consider before you leave Thailand for 5 years!
Hope this helps anyone in the same boat.