Same cooker same problem but 2021.
These upper and lower ovens appear to have 'ignite and sense' electrodes, so it is the GENERATOR which senses when the flame is on, to stop auto spark on flame on.
All electrodes sparking all the time the lower oven was alight.
changed ALL the parts timer, generator, checked earthing.
Answer ? it turned out to be A NEW lower oven electrode. !!! for £25. Easy to fit.
The old one had some 'caked on' cooked meat juices which caused the auto spark generator to think flame not there.
Ed snowden, November 2021