It actually means that an I-plate would do you next to no good!
1. You would still have to use an ADSL filter on each phone/fax/Sky etc connector;
2. You
might get a cleaner & thus faster internet connection - but quite possibly there would be some degradation from the other circuits running round the house.
If you want to put things right, and have the time & energy, you could either,
1. Ask BT or your telecomms provider to wire the master socket correctly or,
2. (what I would prob. do - illegally) rewire the master socket. Info here, for example:
UK Telephone Wiring
On balance, if you're getting an acceptable speed / reliability, you may wish to leave things as they are. Up to you.
Good luck.