The top and bottom ovens are on two separate circuits, so though the top oven is working it doesnt mean power is getting to the second oven.
To troubleshoot you need to use a multi meter and test what (if any) voltage your getting to the lamp terminals, and check on each cook mode. My guess is there is no voltage going to the bulb, in which case it will be a lose wire or potentially faulty switch on the control panel. If there is voltage going to the terminals, you will probably find the terminals are either corroded or bent out of shape so not making contact with the bulb, so its a case of just bending them back in shape.
Since this is a high energy appliance, there is a real risk of electric shock. I suggest you call an engineer out to troubleshoot this problem for you, as this is pointing towards a hardware failure. If you would like me to suggest someone in your area please provide your post code.
Good morning, I would suggest you use Highland AS, who are based in Inverness and can be contacted on 01463 791822
They charged £77 call out, which includes the labour cost for repair, and then it will be the cost of any parts.
The alternative, is to setup a service plan, and I would suggest you use D&G on 0808(###) ###-#### they charge £15 a month, but this will cover the repair of the oven, and give you a 12 month warranty for any further breakdown.