You'll find a lot of interesting stuff in an allotment that tells of its former use - usually as a vegetable garden. Different stuff comes from different gardens and most common finds are bits of pottery, pennies, marbles and clay pipe bits. Sometimes bits of bone from composted meals remnants too. The soil will have been churned by the act of cultivation - depending on how long it's been cultivated - so the stuff will be all mixed up until a depth of two foot is reached - then it'll likely just be clay and bedrock beyond. Have a look at aerial images off FlashEarth to see if any crop marks show - and then dig there!