I have built the pen on my allotment from the sheets of Harras fencing as used on building sites.
At the school, they have the linked fencing like the council use.
I picked it up at £5 a sheet, pretty much new.
I have seen that Wilko do welded wire mesh (not the cheap "chicken wire") at quite a good price. I phoned them yesterday and told me it was seasonal so you'll expect to get some in Spring I rekon.