Exactly.

No licence is necessary if no copyright material is performed (or rather, no music for which PRS administers the copyright?)

But as pointed out above, the PRS can make a lot of fuss, trying to prove their case, and it only takes one error for them to win. That is even more difficult than you may think - even Happy Birthday to You is in copyright, so don't sing it!

There does not seem to be a reliable database of Public Domain songs, and even if it existed, there would still be the arrangement to worry about. This has all been covered thoroughly in other threads.


(By the way, it has been held that you do NOT need a TV licence if you only use it to view video/DVD, and do not have an aerial connected.)