I'm trying to fit the provided snowploughs to a recently bought (new) Class 31. However, I'm not finding the instructions very clear - two arrows leading from the snowplough in the direction of the bufferbeam but, to what, I can't tell. I've looked on the model and can easily see the holes for brake pipes, etc. but I'm wondering whether I have to remove the bodyshell to get to the snowplough holes?

No need to remove the bodyshell, just the rapdio coupling. The class 31 instruction sheet is on the Bachmann Spares' website:
https://bachmann-spares.co.uk/file/F7111-3-Class-31-2020.pdf

If you download it you'll be able to zoom in to figure 2 to see that the snowploughs fit into two holes on the buffer beam, one either side of where the coupling hook would be.