Alasdair, just a few to start off with:-

http://uk.geocities.com/morsenet1/id19.htm
http://www.arrl.org/news/features/2003/10/30/1/
http://www.zerobeat.net/g3ycc/g4raw.htm
http://www.schmarder.com/radios/misc-stuff/cpo.htm
http://www.qrp.pops.net/multivibrator.asp
http://home.att.net/~jacksonharbor/cpo.htm
-- 

Regards
Rob Sutton
rob@rob-sutton.co.uk