The 1988 Newcastle schedule does indeed have a Monarch 757 listed as operating MAH-NCL-MAH for Horizon Holidays on Mondays - MON489 arriving at 1535 and MON488 departing at 1705. I guess this must have been changed in a late tour operator chop-around. There were two British Island 1-11 flights from LGW (BIA2005 at 0630 which w'd to Glasgow for Horizon and BIA2701 at 1045 which did a W pattern to somewhere not listed at the time of publication of the Airport Timetables UK. Horizon obviously had quite some Monday Mahon flying - there was a Novair DC10 from Gatwick, an Orion A300 and Dan-Air 727 from Manchester, a Monarch 737-300 from Glasgow (back in the day when there were three based there, with Monarch doing virtually everyone's Glasgow flying) and many more.
The Orion 737 programme at Birmingham for 1988:
Aircraft 1
Sat KG817 BHX 0630 - REU - 1135 BHX KG818 Horizon
Sat KG2207 BHX 1300 - JER - 1540 JER KG2208 Lewis's
Sat KG875 BHX 1715 - ALC - 2305 BHX KG876 Horizon
Sun KG2465 BHX 0005 - GRO - 0505 BHX KG2466 Intasun
Sun KG807 BHX 0645 - LEI - 1320 MAN KG920 Horizon
Sun KG919 MAN 1420 - LEI - 2055 BHX KG808 Horizon
Sun KG827 BHX 2230 - GRO - 0330 BHX KG828 Horizon
Mon KG835 BHX 0810 - MAH - 2040 BHX KG836 Horizon
Mon KG2417 BHX 2230 - ADB - 0740 BHX KG2417 Falcon/Owners Abroad
Tue KG7673 BHX 0950 - GUT - 1300 BHX KG7674 (guess Orion must have had the MOD trooping flight contract for Gutersloh)
Tue KG801 BHX 1400 - TFS - 2345 BHX KG802 Horizon
Wed KG809 BHX 0045 - PMI - 0630 BHX KG810 Horizon
Wed KG803 BHX 0800 - ALC - 1400 MAN KG992 Horizon
Wed KG991 MAN 1500 - ALC 2150 BHX KG804 Horizon
Wed KG212 BHX 2250 - IBZ - 0440 BHX KG213 Orion scheduled service
Thu KG811 BHX 0940 - LEI - 1620 BHX KG812 Horizon
Thu KG837 BHX 1730 - AGP - 2359 KG838 Horizon
Fri KG819 BHX 0710 - MAH - 1300 BHX KG820 Horizon
Fri KG877 BHX 1400 - SKG - 2135 BHX KG878 Horizon
Fri KG847 BHX 2230 - CFU - 0615 BHX KG848 Horizon
Aircraft 2
Sat KG2413 BHX 0705 - PMI - 1250 BHX KG2414 Redwing
Sat KG208 BHX 1435 - PMI - 2020 BHX KG209 Orion scheduled service
Sat KG2601 BHX 2130 - ZTH - 0635 BHX KG2602 Sunmed (tech stop at QTV on the way home)
Sun KG212 BHX 0755 - IBZ - 1345 EMA KG762 Orion scheduled service/Horizon
Sun KG761 EMA 1455 - IBZ - 2045 BHX KG213 Horizon/Orion scheduled service
Sun KG829 BHX 2145 - FAO - 0430 BHX KG830 Horizon
Mon KG813 BHX 0940 - CFU - 1745 BHX KG814 Horizon
Mon KG2411 BHX 1900 - GRO - 0005 MAN KG2410 Redwing/Owners Abroad
Tue KG2409 MAN 0105 - GRO - 0610 BHX KG2412 Redwing/Owners Abroad
Tue KG833 BHX 0720 - AYT - 1705 BHX KG834 Horizon
Tue KG831 BHX 1820 - HER - 0335 BHX KG832 Horizon
Wed KG867 BHX 0700 - QTV - 1200 BHX KG868 Horizon
Wed KG843 BHX 1300 - RHO - 2220 BHX KG844 Horizon
Wed KG839 BHX 2345 - CFU - 0730 BHX KG840 Horizon
Thu KG208 BHX 0900 - PMI - 1445 BHX KG209 Orion scheduled service
Thu KG2891 BHX 1625 - FAO - 2300 BHX KG2892 tour operator not listed
Fri KG845 BHX 0015 - AGP - 0650 BHX KG845 Horizon
Fri KG825 BHX 0905 - HER - 1840 BHX KG826 Horizon
Fri KG815 BHX 1945 - HER - 0500 BHX KG816 Horizon
There was also an Orion A300 PMI-BHX-PMI on Saturdays and a 737-300 AGP-BHX-AGP on Sundays.
As well as the relative oddity of the Orion 737 flying a Jersey roundtrip on Saturdays (I think Britannia did this for a year or two afterwards and seem to recall some Air UK Leisure 737-200 Jersey flying not long after that), the Birmingham 1988 Saturday schedule also has the Brymon Twin Otter operating a charter to Bembridge (with a W pattern to Luton!) for Warner Holidays. I bet that's a departure point that hasn't been seen on the Birmingham Airport screens ever since!