Outdoor garden pest control – responsibility?

Thought I'd garner other's thoughts before replying to the downstairs flat owner. She has the rear garden flat (the garden is exclusive to her flat). My flat is the maisonette (1st floor and loft floor). The third flat is the front garden flat. Get an email yesterday from rear garden flat owner forwarding her agents email:

Due to activity being noticed within your garden and shed and whilst the tenant was in the garden quick action was required to ensure deterrent from the building. There are three boxes externally one next to the bins and two in the garden, bait has been taken on the first two visits a third visit is due next week.

Activity has reduced and all owners should pay towards.

The external box next to the bins along the side pathway to the side of the house is the electricity box I believe, though this has a door and is closed. The two boxes (or sheds) are within her garden, and the garden is demised as part of her flat. The pest control invoice is £420 including VAT

My question, is this a shared cost? Seems to me as it's a problem in her garden (exclusive to her flat) and not a structural / maintenance issue to the building, shouldn't it be her responsibility? Also the rodents maybe coming from a neighbour's compost which could be another possibility.

Have had mice in my flat before and always paid the full cost and never asked the other property owners to split the cost, as I thought it was within my flat, but thinking about it, it maybe should be shared as they maybe coming via the roof voids (eaves) or a hole in the exterior of the building as the tenants guaranteed they always close the door and never leave food around.

Your input appreciated