The issue is that the producer of the data does not have a .proto definition and just has the .cs files.
From looking at other questions it seems it is possible but is there a tool to convert the .cs protobuf-net definition to a .proto file so I can use the file to deserialize in python?
string proto = Serializer.GetProto<YourRootType>();
Note that if you're using DateTime or TimeSpan, you should ideally use DataFormat.WellKnown - to use Timestamp and Duration respectively - however, this is a breaking change to your existing data (this is done via the [ProtoMember(...)])