I wonder why Shifty never returned in Series 7

As with Freddie(Dad)only appearing in one episode - which in essence was a vision of him only but still a picture, Shifty and his antics could of helped make Series 7 a better one than it was (though I have grown to like it miles more in the last 10 year's than I ever have), par the first and last episode.

Shifty was out of jail by Xmas 1990 but I think one of the few mention's of him in Series7 was he was back in prison by the end of Series 7 but again due to be released. It's a real shame he was'nt in Series 7 I feel as with Jack living with Leonora it could of been like Series 4 again with Shifty "replacing" Jack but in a different way!