Outdated Website
The website is using very outdated layouts and code which not only put off Google, they make visitors want to leave too. This increase in "bounce rate" will cause their rankings to drop as a result.
There are clear problems with their basic SEO too, with no H1 tags across the entire site, and 20 duplicate meta descriptions, which could cause them to be penalised by Google.
They also haven't optimised the homepage information for their target keywords around "garden centre" and "Sheffield", which is the most important step to take when optimising a website.