traditionally planting a main crop of spuds is the way to get a fallow allotment started, it really cleans up the soil and the cover from the plant helps to keep weeds from getting established.
i did this on a nasty bit of ground that hadnt been cultivated for ages and i reckon during all the years that i subsequently grew veg on that patch the soil was in the best condition just after i'd harvested them spuds.
Joy Larcoms 'Grow your own Veg' was my newbies way into making my garden allotment work, theres the web of course now but this book pretty much says it all
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Would this work or am I better off building some raised beds and buying in some topsoil or compo?
its really down to you Hughie (there is no right or wrong with gardening

) but i didnt like doing what all the old guys around me did, treading on the soil that they would be subsequently growing in, all that rows stuff didnt really make sense so i divided my space up into beds with paths in between, over time i picked out the stones from the beds and they made the paths.not 'raised' beds but distinct walking and growing areas.
to make the soil fertile and of the right consistentcy i dug in well rotted manure.
good luck with your new hobby