It was DC-9s for a while, but then worked down through ever smaller aircraft types. IIRC it ended up being operated by Saab 340s.
Moley is correct - BRITISH MIDLAND did operate DC9s (Diamond Service) LPL - LHR from the late 80s until the service finished in the early 90s.
Prior to that, MANX had operated SD360s and the SAAB 340 on the same route.
And prior to that, BRITISH MIDLAND had operated the route with Viscounts and DC9s.