I'm a bit late on board with this one, sorry! I have the same issue, a small (but diffuse) L5/S1 bulge and a smaller one at the next level, L4/5. I also have pain on my left side, due to the nature of the bulge. This was actually caused by physio "cracking" my back when I went to him with a more minor and intermittant issue, although I have partial desication meaning it might have happened in time if I'd done something stupid anyway. My advice would be to quit the running (I have), along with rowing, weights and any forward back motion (flexion). Keep with the McKenzies..there are lots of other things you can do too. This issue is (or can) be too serious to just ignore..so please be very careful.
There's the same old prejudice and misinformation in this thread as always appears in "High Street v Running Shop" debates!
If you know what you want and what's right for you then it is highly probable that Sports Direct will be much cheaper. They only tend to stock a serious range of Asics and Nike though, so if your brand loyalties lie elsewhere, you may be disappointed.
Running shops, in theory, will have more specialised knowledge if you are a beginner or novice and don't know what you need..that cost is incorporated into the cost of the shoes! Ironically, my local running shop has been known to talk what I know to be complete nonsense, whilst the guy at SportsDirect knows running shoes inside out! He did used to work in a running shop though, on the other side of the country!
You are perfectly able to determine what kind of shoes you need without visiting a shop if you read one of the many guides on the internet and in books.
And the person who said SportsDirect give no refunds is also wrong. They only give cash refunds if they have a legal responsibility to do so (ie, the goods are faulty). In other cases they give a discretionary credit note...but at the 2 branches I'm familiar with, it is always forthcoming without question, as long as the goods are returned unused!
To the person who said that size availability may be an issue, correct...but only in so far as they only keep one pair of each size in at a time. If your size isn't there, just come back the next day and it will probably have automatically been replaced!