is it possible he served the fencing order on the vendor anyway, and it's been included in the costs of the block

my understanding is that they can't expect you to pay for something unless you've been served a fencing order BEFORE the fence is erected - you have time to respond with a yes or no answer - if it isn't resolved within a set time frame (i believe a month), it can then be taken to the magistrates court to resolve it. if they build a fence without your go ahead, you're not obliged to pay for it.... and you definitely only have to pay for half of the minimum practical fencing for your area (being a 5foot paling fence if it's a side or rear boundary i think), not half of what they have chosen to erect for themselves.

as was mentioned, you can always dress it up along there for a better look - plants are your friend!