Unfortunately, my ability to rest it was hindered by the fact I'm a single mum and have young kids with additional needs. My GP referred me to a physio for advice, who didn't seem to know the difference between tendinosis and tendonitis, and gave me exercises to do. Another issue is that I should have been given amended duties on my return to work, but this wasn't possible due to severe staffing issues.
This might complicate things - We can all appreciate the reality of being a single parent to a child with additional needs, but your employer would likely argue that you didn't follow the Dr's advice to rest, which could have aggravated any injury.
They would also point to the poor advice you got from the physio as not their fault.
With regards to the amended duties - Did you bring this up with them at the time? They will argue that you agreed to return to regular duties, and if you weren't capable, that you would have refused and provided a fit note from your GP.
From the info you've provided, it seems like the reason you're suffering six months on is not following initial medical advice, plus getting poor advice from your referral. Neither of these are your employer's fault, so I'd be very careful looking at NWNF.
The cause of the accident is unclear though (in terms of what happened and why you had to run), so if your employer was at fault there, you could still have a claim.