as above can the winner of the junior class go on to win best of breed

Yes providing they are not beaten in another class.

As Nova says yes as can a puppy class winner - providing unbeaten - Best of Breed challenge is all unbeaten dogs and any of these can be declared best of breed

Any class winner which has not been beaten can be Best of Breed, no matter how young or old or what winning it's done in the past...for instance from Novice class. :)

What about winner of Good Citizen Class?  Do they go into the challenge if unbeaten?

>as above can the winner of the junior class go on to win best of breed

Yes; my Daisy's litter brother has just done that at Blackpool! :-D :-D

What about winner of Good Citizen Class?  Do they go into the challenge if unbeaten?

Yes, the mother of one of my dogs won the CC from the GC class at a show last year.

One of mine won the CC, BOB and Group Two at a championship show at 11 months out of Puppy, in fact won two of her CC's out of Puppy and the third as a Junior. 

Not usual for our breed, but our entries are not as strong in numbers as they used to be.

A dog in the UK cannot gain it's title until it has won at least one of it's CC's over the age of 12 months and three CC's must be from 3 different judges, but there is nothing stopping puppies or Juniors winning multiple Best of breeds if they are the best in the judges opinion.

Yes, my Henry won the CC and BOB from Junior & Graduate class wins, and won several BOB at open level from Junior. :-)