An uplift clause is becoming more common, at least round here. Him adding a second one smacks of greed, but it's not impossible. If you want it as horse pasture, there's nothing to worry about. It means, if I read it right, that if you buy it for £10,000, and then getting planning permission, for example, and it becomes worth £100,000, you'd have to pay them both 25% of the increase in value (£90,000).
I'd be having an interesting discussion with the first owner about how the cost of the second uplift impacted the value of the land ("It's not worth £100,000, only £75,000, due to the uplift bill...") but that can be sorted with a careful contract.
I have some land with a similar clause on it. I am obliged to pay within 12 months of the value increasing, regardless of whether I sell it, or not. So, getting planning permission would cost me the uplift, even if I never used it.
It lasts for 20 years, and is currently 50%, in my case. Time and values are, of course, negotiable. Depends how much you want the land as it is. My vendor's genius solicitor allowed me to removed a house-plot-sized area from the clause without question.
Finding someone in 30 years isn't really your problem, I would guess. You could do all that was reasonably expected fairly simply, and your obligation would end there.