Dave explained :
I bought one of these little rascals that help you to do mitres in the stuff
http://tinyurl.com/yzhmpor (link goes to Axminster tools) but what's the best
implement to actually cut polystyrene coving with? Wood saw? Hacksaw? Bread
knife? Something else?

It cuts cleaner with a hot wire cutter, but failing that hacksaw or
bread knife using a gentle sawing action.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk