The certificate of compliance, as I'm sure your solicitor should have explained to you, is most likely to certify that the house (or extension) was built in compliance with the planning regulations and/or the terms of the planning permission granted (after the fact, in your seller's case, as it's a retention).
Your solicitor will most likely be the best judge of who should issue the certificate. I would be surprised if he or she allows the vendor to issue this himself, but I've been surprised before. Any costs associated with the certificate should be borne by the seller.