You can shove a Mk3 as far up where the sun doesn't shine as you can. They've let me down so many times (not mine but my mates) making us dig deeper, for longer and for no reason than I ever have with a Mk1. Thing is, with the Mk1 we have total confidence in it's ability and we are NEVER more than a few inches ot on a dig.. We use one at each end of the bury, one in the middle and the next as the locator. Foolproof. Efficient. Gets results. As to an earlier comment about collars, your talking right out of your arse. 30 years of ferreting and you got to be a bit of a judge.