This is one of a series of increasingly tenuous, easy caches celebrating Rickmansworth's musical connections.
This leisure centre is named after Rickmansworth's most famous historical resident (apart from Val Doonican - see Rockmansworth #3), William Penn. He founded Pennsylvania, although it was actually named after his father.
One of Pennsylvania's exports was Bill Haley and His Comets, responsible for one of the huge early rock'n'roll singles, Rock Around the Clock.
Magnetic nano - you will need a pen. There's nearby parking.