Thanks Susan, Tomer.
I have de-waxed and cleaned with the vinegar and salt. I'll let it dry for a few days and see if it looks like cracking. If so, I'll try the oil. I was a little surprised that it did crack as the weather here has been very wet recently.
I'll persevere with the wax at this stage. I've got my first cheddar which I'll wax at the same time. While I take your point, Tomer, in this case I think that the problem was not so much the wax but that the mug that put it on applied it too thinly on the sides.