Hi - good luck with getting your plot under control. It has taken me three years to get into a reasonable shape but worth every minute.
One of the things that takes time is tracking down the cheapest local suppliers for all sorts of things such as seeds, chicken wire, netting and so on. There will be a local allotment society nearby, if not on your own plot, and they will have lots of ideas for you.
Check in the yellow pages for stables and see if they will let you take their horse poo to make your own manure. A local farmer here delivers a trailer of well rotted manure for £20- gorgeous stuff. I will leave more knowledgeable folks to advise you on the bought stuff but I would have thought anything labelled "farmyard manure" would be good.