The definition of motor vehicle is "a mechanically propelled vehicle intended or adapted for use on a road." so in that sense it would include motorcycles.
However, rather than risk getting caught for riding a motorbike with no valid insurance I'd get written conformation from the insurance company that their cover includes motorbikes before risking taking one for a ride on that policy.
It does include motorcycles. The term 'motor vehicle' occurs a lot in legislation and always includes them. You'd be surprised if the law did not require you to have insurance for one, but the law doesn't specifically mention motorcycles !
It would cover you for motorcycles if you were licensed for them and had declared that you were intending to use that policy to cover you for riding them - you would have to read the exclusion clause(s) to see if they were excluded.
IMO