Hello,
It's always a challenge to diagnose and really comes down to a theory so I'm happy to give you mine! First off: it's not going to be due to your robots, sitemap and htaccess etc. unless you've blocked it but it seems you've not!
What it will most likely be is link profile and content. Google seems to think that your home page is the best result for that search term, this could be due to having some of the keywords in that page (especially right at the top!) it could also be your home page has stronger link profile, even more so when specific links are from Edinburgh related domains etc. this is compared to the pages you want to rank which are lacking in links etc.
Now you could possible do some fun local citation shenanigans to help or tweak the content to really give it some clout but really it comes down to tweaks... lots and lots of tweaks! as with most things SEO to get that balance and there is no golden rule, no keyword density that will result in the magic number one spot I'm very sorry to say.
So I say go out, show Google what for and make your pages the best gosh darn garden landscaping page in Edinburgh! This can be linking to some other neato pages that help with your point, get some people to link to you saying "look how gosh darn amazing these guys are" etc. and try not to worry too much on what page is ranking
i hope the above was at least a little insightful, i'm aware its a still a little vague.