That pH is wholly suitable as long as it is stable. Zero KH would drop the pH but that is not happening according to your readings (I expect 6.6-6.8 is typical of your tap water in Edinburgh). I recommend that you don't adjust the parameters. Every water change would be an opportunity for a mistake and as you know, nature stabilises itself without our intervention where we let it.
I do not know of anyone who has succeeded in converting nitrate to gas in a freshwater aquarium. The oxygen levels prohibit it. I think you are chasing rainbows here. Even
@AbbeysDad hasn't achieved this yet and he researches and writes articles about filtration. The way to control nitrates is maintenance (cleaning and water changes) and avoiding overfeeding. Plants help, and of course the water source is important. Do you have nitrate in your water source?
The key is in the biological filtration media, which is purpose-designed. It is made of sintered glass, infused with a long list of trace elements, all with the intent of enhancing bacterial growth, including anaerobic bacteria.
It is expensive, but 1Kg of it will, once established, fully process the bioload of approximately 100l of normally stocked community tank (I think that's roughly 2 lbs for every 25 gallons). These figures are obviously approximate, as different fish have different requirements. A goldfish tank would require double the amount of media, owing to the level of waste they produce, for example.
I set my tank up in February, using mature filter media from a previous tank to kick off the cycle. I wanted to have more biofiltration than I needed, as I'm not comfortable operating at the limits of my capability in anything I do. I have 7Kg of this media in an Aquael Ultramax 2000, which is purely devoted to biofiltration, since I installed a booster filter to handle the first, mechanical stage (the Ultramax 2000 has a pre-filter installed as well). The nitrate levels in my tank did not rise above 20 ppm until June, when I started regular monitoring, and small, partial water changes. They've never risen above 30ppm since set up.
I bought a Salifert nitrate test kit because the API one is a devil to read, and over the last two months I have determined that the denitrifying bacteria are fully established, as my nitrate levels have consistently shown up slightly paler than 5ppm. And, because of all the pre-filtration, I seldom need to open up my main filter, except for routine impeller cleaning, etc. The only time I test for nitrates now is a few days after such maintenance—a cautionary, owing to the sensitive nature of the anaerobes.
I am not alone. In the UK, there is a growing number of aquarists who are achieving a full nitrogen cycle, and I have come across other companies manufacturing similar products. However, I spent more money on the filter and biomedia than I did on the rest of the set up combined. It's not something everyone would be prepared to do, and it does take some financial commitment, but the media is designed to last for many years.
If you'd like to look into it, check out filterpro.co.uk. There are many educational video links on the site, too—it completely transformed my understanding of a self-contained aquatic ecosystem.
I remember when he first appeared with his products, people were pretty sceptical but some members here have used the products and like I say some have had good results others saw no change. My concern around your filter media was when I was thinking you had changed your bio media at some point in the last few months but I know it was your mechanical now. 
