I only use 15' collars handy for thick hedges 8' just would not cut the mustard. Trouble is 15' ferret collars are like rocking horse shit very few being sold and you cant blame lads for hanging on to them. I gave two of them 15' terrier collars to a lad not far from you, way too big for my ferrets.
I must say looking at the build quality of them repo boxes just from pictures they do look better than Deben's Mk1's. Like you said Sean "two batteries which double the power" to give the 15' distance.
Cheers Arry