To be great, amazing, incredible. Unspeakable!
"That game was fablon!"
Brand name of a self-adhesive decorative plastic sheet product, popular in the 70's and 80's. For a long time in the UK, "Fablon" was the only kind available - which made "Blue Peter"'s reference to "sticky-back plastic" all the more amusing.
Cover the cardboard box with the red sticky-back plastic