thanks all for your responses, really useful.
one further question - does making a mayday call ever guarantee a sterile runway?
I guess your answer will be similar i.e. it depends - there are some failures on the bus where once the gear is down, we are effectively committed due to the huge increase in fuel burn and therefore making a go-around almost always a non-starter. I therefore generally recommend requesting landing clearance at 10 miles before extending the gear. Any thoughts?