I never noticed that. Did Mark Lamarr ever cover for Chris? If not he'd be the only Down your Doorstep permanent host not to host the main show.
No, Lamarr never presented the main show, and I don't recall anyone standing in for him when he was on holiday either. Of course, he was presenting The Word at the same time, and it was Cheggers' appearance on The Word that got him the Big Breakfast job, because he was basically there to be laughed at but charmed everyone with his self-deprecating humour and professionalism, and Mark Lamarr told him he'd written a load of jokes at his expense but realised he was going down so well he couldn't use them.
Lamarr said that his worst moment on The Big Breakfast was when he took a camel onto a council estate in Glasgow and just before they were about to go on air the camel started going mad, snorting loudly and running amok, with its handlers trying desperately to keep hold of it, and all he could do was come on and say "Hello, we're in Glasgow and we've got a camel, bye!"