It’s always like that. You search for 2 hours, you eventually end up posting on SO, and 5 minutes later, you find the answer.
In the WCF Options tab of the properties of the project defining the ServiceContract,
there’s a checkbox labelled “Start WCF Service Host when debugging another project in the same solution” that I unchecked.
I’ve no idea how it has been checked in the first place.
Anyway, that solved my problem. See the MSDN Reference for this project setting.