Nanna asked housing assocation to sort rat and ant problem out where she lives, but they turned around and said she has to sort it, does she?

Ex-Pest Controller here. Housing associations have a duty of care towards their tenants. This means that the property has to be kept safe to live in. With the presence of rodents, there is a threat of disease such as Leptospirosis or Hantavirus. With your nan being the age she is, I'd say it's cause for concern. A proper rodent treatment should consist of a minimum of three visits. The initial inspection should be checking for signs of rodents to assess which rodents you are dealing with, as the treatment has to take into account behavioural differences for different species. (Rats tend to avoid baits initially. This is known as Neophobia. Mice tend to be more curious and take to bait quickly.) The second inspection should be taking into account bait take, and following signs to analyse potential entry points. The bait should be refilled if it has been taken. The third visit depends on how the rodents have reacted to the treatment. Usually, the bait used induces dehydration which drives them out of the property in search of water. If the pest controller is able to, they should UV Tracking Dust or Gel to aid in finding and sealing the entry points. Sometimes, it can take more than three visits to sort a rodent infestation. This isn't uncommon. In older properties, sometimes the neighboring property will have to be treated also. The British Pest Control Association (BPCA) are able to offer advice on treatments and may be able to help you understand the legal rights that you have in this situation. I would make sure that your Pest Controller is either a member of the BPCA, or the National Pest Technicians Association (NPTA), as this will ensure that they have the correct training for the job. There's also the option to kick up a fuss on social media, but it could sour things with the housing association.