At the beginning of this thread I think you said you are Buckingham GW rather than GC? If so, your problem is solved - GWR and BR(W) didn't reply to the 2-1 TOS bell code. However, I would still recommend the capacitor and LED. I think it's as simple as a diode off the bell feed onto a capacitor with the LED (and dropper resistor) fed off that. Try it and see, it's all down to component values for length of delay. This simple circuit gives a fading LED, if you want a 'snap' off, you will need a more complicated circuit.
Paul.