Sourced a new pair of heads from someone on Antilag. Complete heads with cams, covers AVCS pulleys etc for $400 which I thought was bargain.
Anyway. I've still no idea why a valve dropped. It certainly wasn't a build issue and I put the timing belt on following particular attentions to the method of rotating the LH cams in opposite directions when timing it up.
Anyway I'm stumped, but this makes interesting reading.
Cosworth official answer after their engine`s large valve head failure - Page 2 - NASIOC