I would contact the companies but frankly I don't have time, we've already got loads from a whole host of companies and businesses. The address they're being redirected from is the company's old registered address and they started arriving around the same time the address was changed via Companies House so it looks more like a mistake than anything dodgy. Plus why would someone choose to get their pension statement redirected to someone else's house?*
I'm guessing the accountancy company is just for being a contractor or something hence no public face or contact details.
* I opened one by mistake because I actually receive redirected mail all the time for my dad as he lives overseas and the original address is normally fully masked so there's no way of telling unless it's got the person's name on, which this didn't. It was a full KPMG staff pension statement including his balance, DOB, account number, yada yada.