Who do I talk to first?

[mvs]

I want to sell a plot of land with a building on it. Who should I talk to about separating it from the title deed and making it available for sale?

[muchlovex]

It would be sensible to speak to your solicitor about this.

[buildersmate]

You will probably need a solicitor to help you divide the existing Land Registry title into two. It is perfectly feasible to DIY, but unless you know how or are propared to learn fast by using the LR website, its going to be better for you.
Having split the plot into two title deeds, then you can talk to a land agent or estate agent.