Hi Pilly,
Have you tried Elite in Rainham? They're not far from Romford, and are pretty good - I usually go there for any tyres/puncture repairs I need. I don't know if they'll refuse to fix a puncture on a runflat, I've never asked as not been in that situation (yet) - but if the screw is right in the middle of the tread I don't see the problem.
Good luck!