The OP has a problem with theft from an allotment shed. My cheap solution:
To protect our shed I installed a 'rape' alarm. You can often get them for a quid (literally) from a pound shop! You know the things I mean, a battery operated alarm which is set off when you pull the pin out and runs until the battery dies.
eg EBay http://www.ebay.co.uk/sch/i.html?_trksi ... &_from=R40
Fix the alarm in a open topped wooden box or similar (I used a old tobacco tin that was lying around) drill a hole where approximately adjacent to where the pin/trigger is & feed the pin through the hole. Fasten the tin/alarm box & run a length of fishing line through the pin and across your shed at ankle level. Theory being that when some scrote breaks in they will trip ('trip over' hopefully
Obviously it won't reset itself but for a quid?
Would also make a great backup alarm for any house/shed/garage etc. In fact I'll re-post this in DIY section.