I made a rule with myself that this story would be just the puppet characters, and then broke it to use the Journey to the Far Side of the Sun's new planet to replace the shrunken Mars, but my plan was to try to write a time travel crossover between UFO and Space 1999 some time in the not too distant future too.

They are my favourite shows of Gerry's, but it was hard to work them into a crossover, because UFO is clearly set in 1980 (unlike all the futuristic puppet shows in the 21st Century); and with Space 1999, once the moonbase was blasted out of earth orbit, the possibility of interaction with other characters was pretty much impossible. 

I haven't thought of anything in either show to build a gts plot from either, but maybe I'll make up my own.