I've worked for BT coming up for 25yrs, but the first 3 yrs were on 'Power & BES' as a sparky. Therefore I can't comment on where the demarcation point of old was. I have to say though, I'd be very surprised if it was at the BT18 !!!! That would mean you were responsible for the 'Lead-in' and the Line Jack Unit ??. Can't see that as being the case.
Fast forward to today, and we (Openreach, on behalf of your SP) are responsible up to the Master Socket (NTE5). If you have the old-style internal master socket (2/1A, 2/2A, 4/1A etc etc), you will not be charged if a fault on the line is caused by any of the extention wiring or sockets in the premises, as you have no way of legally isolating them. With the NTE5, you can remove the front plate (where all the extention wiring should be connected), and plug your phone into the 'Test socket' in the back plate of the NTE5. Obviously, if you have dial-tone in the Test Socket but lose it when the front plate is reconnected, you have a fault on your 'own equipment/wiring'.
As an aside, as well as replacing and bringing your internal Master Socket up to par, we should also change the old BT18 for a new BT66. The underground wires will be 'grease crimped' through to the 'Lead-in' rather than connected on screw terminals. It makes for a better, longer-lasting connection, as oxidisation and a build up of verdigris causes problems on screw-terminal connections.