I like the terrier of the lead, an checking the earth's properly, as mango says, do you take the terrier to every entrance, you would need to if you just kept it on lead, over the years trying earth's, there was a few occasions on rangey spots with a few holes, the kicked out holes dog went in a foot or two back out, done this at different clean entrances, nothing, then finally to the dead entrance, dog enters, an were in!!!
Dog needs to be of lead, trying every hole.