SSD is right.
Practically, there is nothing to stop you, but the law is clear - you need differences training and a sign-off by an instructor (I assume the instructor needs to be suitably qualified too, but...?)
If you have bought a share and try to fly it without such a rating, you will become highly unpopular. Most groups (as mine has) have rules that clearly state you need to adhere to the legal and group rules - and flying a tailwheel aircraft without the necessary sign-off will invalidate the insurance.
If the CAA don't get you, the other members will
