Sounds like the diverter valve isn't sensing the flow of water to your shower but opening a tap is more water so it does. Modern boilers don't have pilot lights, they have electronic ignition so it's normal to not see a flame when they are off.
Derek is a top bloke but even though his phone numbers are on his website I'm not allowed to post them here so I'll post a link to his website
Get him phoned up and tell him us muppets on AccyWeb recommended him
Family Gas Care Company Ltd of Rossendale