I will try and help with some of the points you have raised but it would also be beneficial to buy a book from taxcafe which helps you save various taxes on investment or second properties. It was very helpful for meand well worth the £25.
If your parent's can move into your Grandmother's house for a year or more and send HM Revenue & Customs a letter electing it as their new main residence (keeping a copy and the confirmation for records). They also need utility bills, council tax, bank account statements addressed there in case of a query at a later date.
This will help with Priciple Private Residence relief but they can also get lettings relief as long as HMRC were notified of the nominal amount of rental income. Read the book carefully, make notes and discuss any queries with an accountant who is knowledgable with both rental income taxation and Capital Gains Tax or you could be losing out.
As far as I am aware, the value of the house agreed for Probate purposes would be the base price for computing CGT profits. If the house is in both parent's names they can gat 2 lots of CGT allowance, and 2 lots of lettings relief.
This is very a very complicated form of taxation so please see a knowledgable accountant, not just a mate of a mate and the right person can save you a lot of money.
If the rental income has not been declared earlier for any reason, get the accountant to help you out asap and get it sorted. The benefits of the reliefs I mentioned will far outweigh any tax due on the nominal rental income.
Keep all the receipts for the repairs and improvements. Repairs are allowable against Rental Income and Improvements are allowable against Capital Gains Tax.
I hope this helps
ATM