Folks are paying £1m for modern 3 bedroom flats* around here. Recouping the £3.5m, building costs and a bit of profit doesn't seem too far fetched given location and proximity to city.
If you built to the same density as the flats next door, that's maybe up to 12 flats with a market value of 500k - 100000 each. That makes 3.5m seem not so big, I reckon.
£3.5m still looks an awful lot for that plot of land.