Sorry no one has replied. Quiet community these days, but I also suspect very few people have experience of rooks nesting in their gardens. In this area, they nest alongside a park, and alongside the main road. Probably the safest places for them. I know at least one church yard several years ago had a cull of them! (though different county).
Re yours, very hard to know without seeing the situation. Is the colony half a mile away at saturation point? It's possibly a second attempt. Maybe they did try at the colony but lost the nest for one reason or another?
I'm guessing. Seems quite late though for rooks if they've only just turned up and started nest building.