Interesting - it certainly sounds like an interlock switch problem. You are bypassing the top interlock switch ?

Do switches on the playfield score ?

I'd check the solenoid fuses indicated on a placard in the headbox.

I did see a similar problem once and it was the ribbon cable from the CPU to the driver board. Reseat the connector.

There is only one Interlock switch on my machine, I have disconnected the wires to it and then connected those 2 wires together. When I start a game and try to use the buttons inside the coin door to enter the Diagnostics menu, the message "To use buttons, open coin door" is displayed on the DMD which seems to indicate that the Interlock switch has been bypassed successfully.

Yes, switches on the playfield score.

What do you mean by "check the solenoid fuses"? 

I'll reseat the connector tonight and see if that makes any difference.

Matthew