I worked in real estate for a long time. The only way to not know about a toxic problem on the property is for the seller to lie about it. They're required to disclose such things prior to the sale. My guess is the seller either lied about or did not themselves know about the extent of the problem. Time will tell.
I agree with konceptual99, they have some smart people working for them. Even the GB aren't stupid enough to buy known contaminated land with the idea of making money by cleaning it up.