Dealing with contaminated land

Whose responsibility is it?

This depends.  Local authorities are responsible for identifying sites that are potentially contaminated. 

However, if a site is to be developed as part of a Planning application, it is dealt with under the Town and Country Planning Act and it is the developer's responsibility to make sure the land is suitable for its proposed use.

If there is no application and the local authority has identified that the site had a former land use that could have potentially caused contamination then it needs to consider whether this may present a risk to human health or the environment and it will require further investigation.  This is known as an investigation through  Part IIA of the Environmental Protection Act 1990 when the site may not be suitable for its current use (rather than it's proposed use, which, as mentioned above, is dealt through the planning process).