His death is briefly reported in the August 2008 issue of Stamp Magazine: Clive Abbott was the first artist whose name was printed on a postage stamps of the United Kingdom he created.
The artist and printer's signatures on British commemorative stamps lasted only four years: began with the two stamp series for the opening of the Post Office Tower de Londres in 1965, it had already been abandoned when Abbott depicted Ralph Vaughan Williams in action for the 1972 centenary stamp.
A surf on the catalogue-site Collect GB Stamps helped me find the final signed series in January 1969. But the search for printing differences between printers of British definitives is still practiced in Great Britain.
Note: in Spring 2007, Cross Post, the Journal of the Friends of the British Postal Museum & Archive published an article on Clive Abbott and his British stamps. The article was recently reproduced in Gibbons Stamp Monthly.
31 July 2008 update: in August 2008 Gibbons Stamp Monthly, Jennifer Toombs wrote an obituary for Abbott, with whom she designed two omnibus stamp series for the Crown Agents (25th anniversary of Queen Elizabeth II coronation and 21st birthday of Princess Diana). The philatelic artwork of Toombs is the topic of an article in the same issue.