Op I have just been looking into all this and I found this yesterday
www.telegraph.co.uk/finance/personalfinance/11879330/How-to-prevent-family-and-others-from-challenging-your-will.html
Re Capacity
If you are aged 70 or over, or have a history of mental illness, it might be worth asking your GP for what is known as a “capacity report”, which states that you are of fit mind to write a will. Your solicitor may ask you to do this, even if you feel it is unnecessary.
“If the will is challenged down the track and a claimant says that you were not mentally capable of writing a will, this document will go a long way to fighting the claim,” Mr Evans said.
Ask for copies of your solicitor’s notes, to ensure you are satisfied with the level of detail on record.
It is also worth writing a “letter of wishes” to add to the file. This explains, in your own words, who you want to benefit from your estate. If you want to exclude someone, include specific reasons why. “Avoid spiteful language here,” Mr Barnett said. “The court will not look favourably on it. It is much better to couch things in positive, factual terms.”
Either they do have this capacity back up - or they dont, if they dont, looks promising you may have a claim!
This behaviour does not surprise me one bit - we had very similar in our family, when people move quickly like this its very hard to un pick and back track,
Another thing you can do is put caveat on will and do you have copy of your grandfathers will?
I am pretty sure you would get something and should contest it - for principle apart from anything else. but I have no legal expertise. Its the fact your GF will was split in such a way...its all so shifty