Prompted by the latest story to appear on the BBC website here
One of the issues about the widespread use of 433.920MHz with paging for car alarms and locking is that there are other devices using this frequency. They rely on the short duty cycle (you press the button for half a second or so) so that you can have multiple users, it's a crude TDM (Time Division Multiplex) if you at it that way. All you need is a unit to lock up in transmit and you could cause a problem.
The second issue is that in order to keep the cost down, the receiver technology is deliberately kept simple. There are regenerative and direct conversion receivers in a lot of devices. They are simple (and cheap) but lack selectivity from signals on nearby signals, consequently a strong signal a few MHz away would desense the receiver, which is probably what the case is in Southampton. There was a similar case in Windemere, in England's Lake District, caused by wireless terminals in a restaurant.
In the UK, Amateur repeaters in the 70cm (430 - 440MHz) band are located between 433.000MHz and 433.375MHz and that's near enough to cause problems in such as location as Otley, West Yorkshire, where the repeater antenna is on the roof a a pub (with car park) and it is rumoured that there have been callouts to motoring organisations when they've failed to unlock or disarm their cars. The input frequencies in this band are 1.6MHz up from the outputs in the UK, but you'd only get a short outage from a passing amateur.
TETRA could also be an issue, there are lower than the Amateur allocation in the UK, but a strong signal strength would produce the same desensing effect.
Just to remember that these low power devices get the third bite at the frequencies, the MoD being the primary user, Amateurs are the secondary allocation in the UK.
The third issue, in that in the race to keep the costs down, the frequency control is often just a ceramic resonator or SAW device and they use wide modulation on FM or just AM, and a similar pass band on the receiver doesn't help selectivity. No they're not going to employ superhet receivers with crystal filtering for this application - it'll up the costs and the complexity somewhat.
One word of advice, if it's a critical system, or you require it to work all the time then 433MHz is not the solution.
After all, hopefiully you can put the key into the door to open it! Not so with some new cars though it seems.
