Mafioso
Please listen to what Rodney & Atjan are suggesting. I'd also use PIRs in favour of beams, but the idea is you want your alarm screaming as soon as your premises are breached/ entered. Install electric fencing and link it and the PIRs to your alarm & alarm to armed response. Partition your garden into zones so they can be independently armed and so you know where the breach has occurred. Nothing will keep the fckrs out, but this way you'll know as soon as they attempt to enter.
btw, do you have an active Community Protection Forum in your area?