I went to a physio last night and he diagnosed it as a muscle tear, but nothing too serious. Said no running for two weeks and ruled out Wasdale (based on the distance alone, I hadn't mentioned the mountains). Still keeping a glimmer of hope that I'll be ok for it, but not to worry if not. It's hardly Andy Schleck missing out on the TdeF!
Don't imagine there's many fell runners in these parts, but I'd really recommend the place if there are any:
http://www.loughboroughphysio.com/
I saw Dave and he was great.