I want to make my own worktops. Should I make them in sections or in one piece?

This depends on the size and configuration of your project. Any lengths of worktop over about 3 metres will need to be in sections or you are prefabricating your worktops and then installing them. If you want a seamless continuous effect you will need to cast the concrete in place or insitu.

Bear in mind that in the UK the largest size of most standard sheet materials is 2400x1200mm. Anything over this size both becomes more difficult to source and becomes more expensive.