i bought one that looks like a square cast iron hedgehog off fleabay for ÂŁ20 works great in block / brick / some stone walls - didn't like Cornish granite much - just bounced off but for ÂŁ20 it does save a hell of a lot of time and grief, drill 7mm hole- set sds to roto stop and and bash away- on average in block / brick about 60 - 90 seconds to do a 1 gang 25mm box and not too bad for dust, most times don't need to do any remedial work around box it cuts that clean, great investment