I do not understand how the farm can have been left to the son (unless this was under your grandfather's will) if there was no will. Nor do I understand how he can have used the house as security - unless of course he was the administrator of the estate and the bank loan is for the payment of IHT - although if it was a working farm there should have been agricultural property relief (APR) and business property relief (BPR) which may reduce the IHT bill.
This sounds horribly complicated. I really think you need specialist advice. Broadly speaking though, your grandmother's verbal wishes count for nothing at law, although there is potential there for an estoppel argument that too needs specialist advice. Yes, you can in certain circumstances force him to sell - but won't that leave your mother without a home?
You can however do a search at the Land Registry to find out if the land is registered (in which case there will be no deeds). You can also do a search at the Probate Registry to find out if a Grant (this is the document that allows an administrator to act) and to whom it was granted.
You really need a solicitor who is either a member of the Probate Section of the Law Society, a member of STEP or a member of ACTAPS.