Council of Mortgage Lenders
This site will set out post-completion requirements of lenders within their CML Handbook.
HM Revenue and Customs
This is the site for all stamp duty enquiries as well as any other taxation matters that might arise post-completion.
For all post-completion matters relating to registered land and access to the Land Registry Portal.
National Association of Estate Agents
Of benefit should a dispute arise over the payment of the estate agent's account.
The Royal Institute of British Architects
After completion the client might want to carry out alterations, additions, or a complete reconstruction. If so, an architect will be needed.