Sounds like the pest control bloke is failing to truly identify the source of the infestation and just dealing with the problems in different properties. If there is a serious rat problem in an defined area there will be a burrow or nesting site fairly close by. As somebody who works in pest control I would recommend that your parents find a pest controller who is willing to carry out a proper survey of the whole development and try to track the activity back to the source. If he isn’t prepared to get muddy and crawl through hedges and old sewers he isn’t doing his job properly.
Does the development back onto any older properties or an industrial estate or even a patch of waste ground? Sometimes a pest controller has to venture onto other properties and ask difficult questions of other land owners in order to figure out where a problem is coming from.
Sometimes a longer term management arrangement needs to be entered into in order to reduce the incidence of rodents entering sensitive areas and buildings but the industry is slowly moving away from just constantly placing bait out in the hope of killing rodents. Perhaps your parents and their neighbours could enter into a collective arrangement to fund a comprehensive short term treatment programme to cut the rat numbers right back; if they are calling out the pest controller separately he may not be seeing the bigger picture.
You may find the local council no longer provides free or subsidised pest control services; many don’t these days. The BPCA will be able to recommend a local independent pest controller that they have audited. If they are in North or Mid Devon they can always give me a call 😉
Happy to give advice if its needed, but not tonight, its late and I’ve got an early morning appointment in a loft trying to catch rats. Its a glamorous life…