Welcome Dave!
You can pay BT Openreach for something called "change point of entry" and then they will put a direct clean run from the outside world straight in to a new master socket parked wherever you want it. I paid about £120 for this but that was some years ago. It’s per-hour, but it’s normally only less than an hour anyway. The result for me was superb.
If you decide you want to go for it, you should of course tell the Openreach engineer what you’re trying to achieve, maximise speed and reliability, get master socket in the right place and get rid of extensions. Direct engineer exactly as to where you want the run to be, and I would advise to run copper round the outside of the house if needed, so keeping it outside the house and so away from interference sources.
If it is possible, you can get an even better signal, very slightly, by shortening the DSL line even further an having the master socket at an inconvenient point where the line first comes directly to the house and then park a modem there then run Ethernet cable to wherever you want your router or else use wireless networking instead of Ethernet cable. If this would mean that your wireless access point (WAP) or wireless router is in the wrong place for central positioning to maximum coverage, then Ethernet cable to the WAP is again your friend, giving you the best flexibility and the best if both worlds - wireless where it makes sense and modem right at the master socket at an ‘inconvenient’ point.
My WAPs are parked in various places and I have two in the centre of the house and my four ADSL modems are right where the dsl copper comes in, by an upstairs window, with Ethernet linking WAPs to my main switch upstairs which is next to modems and my router.