As with most things you get what you pay for. I use a £ 600 Stihl one for the allotments and cricket pitch edge but luckily it belongs to the village and I'm one of the very few who ever use it! They do smaller models but it will incrase the capacity if you can fit a brush cutter blade to it. Piddly ones won't tackle brambles etc. You could try suggesting to the allotments that they get one to hire out.

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Paul