I have signed contracts to purchase land, the vendor has signed off, the money has been transferred, but my solicitor is still holding the money as the vendor didn't have a PPS number.
The PPS number was flagged over 10 weeks ago, and there is no sign of the vendor's solicitors completing the sale.
How long more should I realistically wait, and would I have a cause of action against anyone for the delay?
"would I have a cause of action against anyone for the delay?"
You haven't suffered any loss - just irritation. Do as the last poster said. Ask your solicitor to set a deadline after which you will seek to walk away from the deal.