I am glad I have come across this thread as I have also had this problem for the best part of 10 weeks which caused me to withdraw from London. I had tendonitis diagnosed by way of ultra scan and have been able to cross train but have not ran. However, apart from the fact I can sit down a little more comfortably now I seem to be a million miles away from running.
I have been seeing a chiro who is working my very stiff S.I joint and I have regular massage but nothing appears to be having a great deal of effect. If anyone on the forum has found a 'miracle' cure I would love to hear it!