[Arran]

 The old photocells were likely selenium based anyhow, so just like the selenium rectifiers they will likely have a shelf life, and would have a higher forward dropping resistance then a silicon based photocell so likely less sensitive as well. I had read about using a photocell from a vending machine coil acceptor in place of the selenium units, which is probably the best solution in the long given the cost.
Regards
Arran