Whilst I can't provide you with any lineside location reports, I would suggest the best place to get many of them very quickly would be Stratford International or Ashford International. From what I've heard, there's to an awful lot of these things running every hour, so going close to Central London is probably best for just ticking them off for sight.
http://www.trainspots.co.uk might have some locations reported on it, I would suggest popping onto there and seeing what comes up if you're interested in finding a good linesiding spot.
In fact, I'm going to have a look myself as I'm interested in going there when they start running in passenger service.