Thanks for the reply.
All plugged into the master socket with a brand new faceplate and shielded rj11. My house is on the corner of a square shaped estate, with my neighbour (one house closer to the cabinet) having an attenuation of 13.6 (compared to my 21.3). He's a minimum of 480m from the cabinet. I guess it's possible they've cabled me the long way round the estate.
But I'm just still surprised the BT checker is so far off as I thought the phone number search would be accurate, but maybe not.