What did the solicitors say when you contacted them, could it be a typo in the address or them using an old address (possible if they use a case management system).
I'd suggest speaking to them again, if you get no joy try the head of conveyancing and mention you want to make a complaint if you don't get anywhere. If you don't get any joy there ring the Law Society complaints section.
http://www.legalcomplaints.org.uk/home.page
Do you recognise the name, who it's addressed to? Does anything else on the letters relate to your property other then the address?