Illegal construction on my land, what should I do?

They cannot build a building without district council approval
district council can sue them

You need to get a solicitor. He will verify you own the plot and check the boundaries of the land. Then he will serve an official letter to the person/address where the illegal construction has taken place. If the people responsible for building on your land don't comply, the case goes to court where it is dealt with and whatever structure has been built can be taken down.

Note this is a long process (years), especially if the other party don't want to co-operate.