Just to let everyone know that everything now seems to be working fine. I changed the 330 Ohm resistors to 1k and everything worked fine. I then switched back to 330 and again everything continued to work fine, so the problem remains a mystery. Before the problem finally disappeared I had rebuilt the circuit 3 times, so that practically puts a wiring error out of the question, especially with the simplicity of the circuit.

Many thanks to everyone who helped out.

Matthew