How can I fix the grill on my Hotpoint BD31 double oven ?

Both elements of the Grill (part of top oven) are not working - no heat at all is coming out, either on "full" or "half grill" setting.

Yet, both grill elements appear to work OK - and certainly get hot - when the top oven is used or when the plate warming setting (in top oven) is used.

Initially I thought the grill elements had gone - but both at same time ? And why do they work when top oven is used.

Any thoughts on what the problem might be ? Many thanks.