Got a tip-off from another allotment holder (who had contaminated manure last year) about a source of good manure. Went along today to a local stables and got a car-load of the most wonderful stuff, about 5 years old, really dark and full of worms- and free! And they've got an enormous mountain of the stuff. Just kicking myself for not getting it before I put my spuds in, but everything else will get a treat this year! :)