As has been said Air Wales did a morning and evening rotation from CWL to MAN. It was in 2001.
A British Airways franchise (I think it was Euromanx) also did a twice a weekday MAN rotation from CWL, using J 41s I believe, but it came off at the end of summer 1997. I think it may have only been running a couple of years or so prior to that and succeeded another operator who tried the route earlier in the 90s. That may have been Business Air or Kilroe but I can't be sure.
Air Southwest have never operated PLH-CWL-MAN. Since their inception they have routed their north-bound flights from Devon and Cornwall via BRS and currently operate a 3 rotations a day BRS-MAN service with DH-8-300s (with two of these daily rotations extended from/to PLH). The route seems to be doing very well in terms of pax numbers.