Manure is the best addition as long as i rotted, it is also the cheapest compared to buying fro garden centres as most allotments get it delivered in bulk. The only trouble is that there is no gaurentee that the manure is weed free. I could have weed seeds in or if it has been stored, there is a possibility of bind weed etc. and i doubt the farmer will want to tkae the responsibility of saying if its weed free or not.
A neighbours allotment gained some bind weed from the addtion of manure last year.

If you get compost from the garden center instead, since it is the least likely to have more weeds, you will probably want to add some blood meal or fish meal to bump up the nitrogen content a little. Some brands of bagged compost can rob the soil of nitrogen to support their further decay.

Hi Sally you really need to start making your own compost. I agree with Nicky that well rotted horse manure is one of the best thing you can dig into an allotment. As you clear your plot dig in the manure, or your own compost if you have and then mulch with straw, compost, even cardboard if you are desperate. This will help to feed the soil.

Sally, can you get spent mushroom compost in your area, if so you can buy it by the ton, but it has chalk in it which will make the soil alkaline.