Have you seen the accounts that the executors are maintaining? (Isn't there an obligation to keep them). If not, then maybe ask to see them. You may find though the low probate value was deliberately too low (several reasons why people do that) so may turn out to be more, or maybe the estate had liabilities you don't know about- big funeral bill, tax bills, debts, mortgage, equity release element. Or maybe the house was overvalued a few years ago- maybe it was just a rumour that got exaggerated.
It is possible they have given their mate a good deal- although as he hasn't officially bought it, you may be assuming they have a gentlemen's agreement to sell it to him cheap when the transaction does go through so they can then split his profit. Would make a good plot for a story.
I know much smaller estates that have taken several years to resolve - one small issue can hold them up- and by the time they are resolved solicitors have taken most of the money in fees.
Roughly what is your wife's share? I'm wondering because if it one 12th then maybe you are only worrying about the difference between say £50000 and £70000- so is it worth spending a lot of money to get back between £20K and £0