460 Electrical problem?

Hi, firstly this is my first post ever and i'm pretty desperate for some help and guidance. I have a Volvo 460 1.9 turbo diesel.
I switched off the car and went to the shop. On returning to the car and switching on the ignition all usual lights came on apart from the glow plug light. When I turned the key nothing happened. When I switched the egnition off the dash lights remained on. I disconnected the battery and this sorted out the lights on the dash but when I turned the key to attempt to start the car again the same happened. I got a breakdown company to get me and the car home but the mechanic said it may be something to do with the starter motor. He seemed as confused as me. I do have a donor car on my drive so I can swap the starter motor but this looks very complicated. Does antone out there have any advice. I don't want to spend all day tomorrow which is a bank holiday messing around with the starter motor if it is a simpler and easier to fix problem.

Thanks in anticipation.

Matt.