This would seem about right. However, an amply clear warning (which would act as the deterrent) would probably work to discharge the duty to trespassers. There might be issues with creating an environment unfit for habitation, however, which I'm guessing the landlord wouldn't be too happy about.
The law says that you have to protect trespassers once they are on your land - a sign saying 'danger - no trespassing' doesn't cut it, because there are times when somebody would be unable to read the sign (say, it's dark, or they're stumbling home blind-drunk from the pub) or would be reasonably expected to ignore it (the classic boy retrieving his football example). Once they are on your property, you have a duty to take reasonable precautions to guard against any danger which can be reasonably forseen, on the condition that the presence of a trespasser can be reasonably forseen (which I think would count in this situation; it is not unreasonable to suppose that you might be robbed)
Similarly, I'm not entirely sure how evident pre-meditation would go down when making an argument of self-defence, so it probably wouldn't be too wise to be too vocal in finding something to hit people with.
I am - very, very badly indeed.
I'm fairly sure the English law is close to that of Australia in this respect, and if that is so, a duty to prevent damage via the criminal conduct of third parties is not owed unless there is some special relationship of vulnerability. If a tenant is not allowed to take reasonable measures to prevent burglary without the landlord's consent, and the landlord then does not consent to the tenant taking such measures, the landlord is in control, and the tenant in a position of vulnerability.
That sounds like English law; I can't quote it but that does sound reasonable.
The landlord is aware of a reasonably foreseeable risk and is the only person who can take measures to guard against it. In that situation, a duty in all likelihood would arise. If the landlord allowed the tenant to take such reasonable measures, that's a different story, because they are no longer placing themselves in a position of complete control. So in this situation, if the landlord isn't allowing bars to be put on the windows, or any other reasonable steps to be taken against what is clearly a reasonably foreseeable risk, a letter reminding them of their potential liability in this regard would not go amiss, particularly accompanied by an expression of willingness to pay for the bars yourself.
In which case they would be legally liable for a robbery which bars would have prevented. However, I still hold that it's most likely the cat from down the road. I have a crow that's recently moved near my house, and he has a habit of repeatedly flying at speed into my windows late at night - I've contemplated borrowing a TI sight and setting up a sting on the little blighter with my air-rifle, but not sure whether the satisfaction would be worth it... especially with the weather we've had lately.