Trying to think what we had when I was at BMA LHR when we started the IOM with 2 x 73 seat Viscount flights a day
IOM 0730/0830 LHR 0900/1000 IOM BD301/302
plus the evening peak time flight in and out of LHR around 1830 or 1900 iirc
We took the slots from BA
Also we had 3 or 4 Viscount flights a day LPL-IOM (that also connected nicely with the 4 or 5 daily LHR-LPL services usually)