[David Burn]

Mountfield 1538h: won’t start. Battery fine. When I turn ignition key I Just get continuous noise which it normally makes when grass collector is full. Less than one year old. Any ideas?

[doganjo]

Mountfields are notorious for that.  I ended up kicking mine once.   

Check your safety switches are OK, and make sure the latch at the  back is in the right position for having the grass box on.

Are you sure the battery is charged?  I once had a faulty battery charger thatn hadb't actuall charged teh battery and it made a noise like that.  Shop replaced the charger and charged the mower up for me and it started fine.

[Ghdp]

Clean the grass box area especially, should you have this,  around the sensor bit that looks like a down turned hook.
Mine did this a lot and When I investigate it  is usually not entirely free of grass cuttings. Seems abnormally sensitive to cut grass given its primary purpose in life. Other than that it could be the grass box is not sitting exactly right.  Hope that helps.