As Chris has said above, the contesting is probably not aimed at you, but more the other parties involved that are set to receive considerable sums. Id just sit tight and see what happens. Its unlikely that your inheritance will be revised given the releatively small ammount you are set to receive from the whole estates valuation.
If you get it, then good, the £1000 from your grandad can go to his great grand daughter, as I suppose youd just put it in savings for her or something.
If you dont, then its no massive hardship, and it substantiates your not engaging with your father for these years.
If you chase it through the courts, then solicitors fees will outweigh the £1000 within a couple of letters and phonecalls, so its not worth it.