Martin, there is no official tuggy rating in this country. Basically a PPL is all that is required.
Individual clubs will set their own minimum requirements however. That may range from just a ppl, to ppl with gliding instructor rating and x amount of hours power time as well as gliding time. Most clubs however tend to sit somewhere in the middle of that ie. ppl and 'some' gliding experience, and possibly a tailwheel rating depending on the aircraft. The most common tugs are Pawnee's, Super Cubs and Chipmunks(converted). There are some tricycle types too such as Robins and Rallye's.
In saying all that, there are some countries in Europe where there is an official tuggy rating. Your ppl will require to be endorsed by the local CAA.