Going back to the OP, try making enquiries with your association leader about better fencing. We've only just moved to our site in the last six months, but a year or two ago a couple of the more motivated and civic minded plot holders managed to secure city council funding for an eight foot metal fence. It's green, quite unobjectionable and has sharp bits on top. Money does exist for these things - it just needs finding! Consider your local city or town councillor as well - handily for us ours has a plot
Secondly, be very cautious when dealing with any intruder. Despite that fence I disturbed a group of teenage boys attempting to steal a duck from a neighbouring plot and knew that I had to be careful what to do. In the adult v. youth confrontation the police, courts, lawyers etc won't give a (ahem) duck what the context or situation is. So stay away. If you enter into any kind of conversation, just make it known that you are not alone (regardless if it's a lie) and make it clear that five youths may outnumber you, but five allotment gardeners will outnumber them. Call 999, and if it doesn't threaten your own safety (i.e. they don't notice you doing it) photograph them with your camera or cellphone.
The single biggest thing that (I reckon) has made our site safe and secure is that there is a diverse mix of plot holders: people are about almost constantly from sunrise to sunset. A civic pride in the communal areas of the plot helps this: a healthy and lively association can bring people to the site for more than just their own plot, and that makes a difference.
But number one tip, get onto your association leader and council representatives to find money to make your site secure. Report every break-in, no matter how minor. If the police are notified of repeated incidents, they have a duty to deal with it.