This is one of the problems of buying ex-council flats in blocks. Over time they are likely to need a lot of expenditure on repairs. When they bought the flat the lease will have included a provision that they had to pay a service charge, which will include a proportion of the cost of repairs. You need to look at the lease to see the exact requirements. The Council should be sending them an annual breakdown of the costs which have been incurred, and this should reconcile to the amount they have paid each year.
There is legislation covering what landlords have to do if large expenditure will be coming about. Look at www.lease-advice.org & phone their helpline, but you will probably find the Council has followed all the legal steps properly.
Some Councils allow people to pay off these large sums over a fairly lengthy period without adding interest, but this is at their discretion and not a legal requirement.
You could contact your parents local councillor and see if they can help, but unless there is something wrong with the Council's calculations I suspect that eventually they will have to pay.