I personally prefer the smaller ferret for working but thats my personal opinion, I also prefer to use one ferret at a time in the ground, a good experienced ferret shouldn't have a problem shifting the rabbits. If you just was over the bury with the ocater you should be able to tell if the ferret is on the move or not.
I hope this is a bit of help to you i know its not a lot,
I got a question for you, you say you have castrated hobs, How hold have they got to be before you can get them castrated, I kept a hob back out of my litter this year to have him done.
Where in south wales are you