Though I should give recognition to good service when it occurs, perhaps others might like to add any names they know
I ordered some helicoil 5/16 BSC inserts 2D (that's the length, two time the diameter) from Uni-Thread
http://www.uni-thread.com/, ordered 16.00 on the12th (yesterday), they arrived with today's (13th) post early afternoon.
Their site is worth a look as they sell much more than just thread stuff these days. perhaps not the cheapest but they carry a larger range of cycle thread stuff than others I think.
No connection with the firm other than a happy chappie