Hi,
I'm at the other end of an identical (every detail) situation so it could even be my apartment you are buying (D15?). In my case my solicitors just do everything at a snails pace. It's an arcane process and IMHO needs to be reviewed top to bottom but that's another day's thread. It could be that your seller is in the same boat as me and everything is happening slowly and because they are not in the apt or moving somewhere else there isn't the same sense of urgency as there is for you.
I got a "you'd better hurry up" call from the estate agent this morning but I'm not even going to ring the solicitor bceause in my experience these things take as long as they take and solicitors always have some excuse/reason why things aren't moving at the pace you want.
I know that's a bit apathetic on my part but I'm not going to waste the phone-call to be told blah blah blah by a solicitor.
Hang in there and good luck.