Do I have to sign an Easement by law?

I have received the following from a developer who sold me the cottage in 2011, which should have been made clear to me at the time - hence the email below.

"The developer of the estate would be very grateful if you could help with completing a drainage easement required by Cornwall Highways in connection with the final section of road adoption under a Section 38 Agreement.

There is a section of surface water drain that runs from the adopted road across the parking areas to [address removed] where it runs through your garden to the river. The plans attached show the line of this.There are about 14 plots which are affected by this and technically all the owners have to sign an Easement document as per draft attached. Highways want this signed to prevent a possible claim for compensation if and when the drain has to be dug up for repair. Only [address removed] is affected from the drain running through your garden and I have been asked to ask you first to cooperate with the developer and Cornwall Highways in signing the easement.

If you wish to use your solicitor please advise the contact details. Your reasonable costs should be covered".