Its another Mike Noble beauty. Eden did the other one in the TV21 special.
I always wondered about this one Bill - when I first saw it back in the day my gut reaction was that it's definitely Mike's work but looking again at the foliage in the background I feel it lacks a certain "definition" i.e Mike would have made the fronds and leaves more pronounced than appears here. A slim difference I know but these days I reckon it's Eric Eden - would love to know for certain tho'. I know Mike's memory is notoriously bad for such things but I'll give him a shout.
It may have also appeared in the Battle Breakdown pages in the 1969 TV21 annual Mike. Had a look at GACCH on Wayback Machine but no joy as to the Hover Tank artist https://web.archive.org/web/20160402203446/http://technodelic.pwp.blueyonder.co.uk/Annuals/Annuals02.htm