Thames Water have some great info regarding sewers versus drains and whether you need a build over agreement (or need to obtain permission):
https://www.thameswater.co.uk/developers/16014.htm
Note last time I had a project with a build over agreement required it added £12k in costs to the job.
https://www.thameswater.co.uk/developers/16014.htm
Note last time I had a project with a build over agreement required it added £12k in costs to the job.