Not good Tony TBH.
I have had e-mail details leaked in the past and other that spam stuff coming through it has been nothing more than a minor irritation.
Given they have (potentially) leaked proof of address stuff - this will only leak to your current address?.
National Insurance info - specific to you but I get why it's a worry.
Contact details - again, specific to you at your current address.
I would be concerned if it was me, and I would be communicating my concerns, worries and suggesting to them that you would be referring the matter to the ICO.