I don't know about Monarch scheduled flights out of MAN or BHX, but I've noticed that 757's are frequently used on the scheduled network out of LTN, including AGP, GIB, TFS, MAH and the new route to Arrecife. My flights from Luton to Mahon & return have been on 757's for the past 3 years.
Wonderful aircraft, amazing thrust/weight ratio, will be very sorry to see them go eventually.
And for the record, MON scheduled still has to be one of the classiest operations around for the price!