Have they stayed with the same agent through that time? what feedback were they given?
I think it needs restyling to be honest to attract more viewers and new photos when the garden's more inspiring.
I'll be brutal, not to be rude, but to try and give independent perspective.
The size of the house looks like it's a home for families, but young families are going to look at the house as it is now and think that it needs a complete overhaul. It looks very old fashioned to me and as though nothing about the place has kept up with the times. If I went to view it would be with the idea that I'd have to gut it to get it to my own taste. Storing about half of their furniture and investing in some more modern duvet covers and accessories could transform the impression of it for not a lot of money. Presumably low offers suggest the viewers have felt the same?
The garden also looks very small for the size of the house and the fence looks neglected and in poor condition.
They could certainly power wash and repaint it and get some new photos with some pretty tubs or bedding plants and that wouldn't cost too much.
I'd probably also change the sellers' comments from what they'll miss to accentuate the positive - this is a really spacious and well planned/easy to live in family home with access to lovely open space. Despite having all the good schooling etc. facilities close by it's a peaceful neighbourhood blah blah
It's no crime to have an old fashioned, cosy home that suits the people who live there, but if they want to move it on they need to try every trick in the book.
Stamp Duty on a £249,999 property is 1% (£2,499.99)
on £270,000 it's 3% (£8,100)
Rather than lower the price they could try advertising that they'd either pay the duty or offer £5,500-£5,600 separately towards stamp duty as an incentive, and that wouldn't affect the sale price as it'd be a different transaction. They'd just need to make it clear that would be subject to a specific sale price being achieved.