Sorry for the delay in replying, but the engineer's inspection revealed a blocked injector.
I did try fully rich but it gave a further drop in rpm. The Alt Air was tested on the run up before TO and it gave the normal slight drop. The fuel was from a regularly inspected supply, tested during the A check that morning, the aircraft had flown approx 2 hours prior to the incident on the day and the aircraft has been fairly busy over the last few weeks.
Thanks for your responses - I guess it answers the original question about ice!
cheers
SP6