Super easy fix. Take the underside cover off, on the bigger lever there's a tiny selector arm that runs in the ratchet mechanism, the pivot on the selector arm is seized and needs lubing up with some thin spray lube. You may need to work the little arm backwards and forwards to free it up. It should work a treat after a good dousing in lube!

Thanks mate. Been working it loose for 10 mins and both levers are nice and free but it's still not ratcheting. Any ideas? Cheers

You need to douse the small selector arm and work that with something small and pointy rather than the large shifter lever.
lots of spray lube is the key!

Yeah. Wot twohats said. I just pop the cable cover screw off and fill it with GT85 until it drips out. I've fixed loads like this. If that doesn't work it's generally knackered.

If you haven't checked already check that the cable inside the shifter isn't kinked. I've seen quite a few where the cable has got kinked about 10mm before the nipple. The cable then jams hte rachet mechanisim.
Pulling about shifters is do-able. But you need a good quality small philips screwdriver and to work slowly when pulling it apart. There are couple of springs ready to ping off across the room.
Worth a go if you think the shifter is bust.