flashy oil light=low oil pressure, so either a duff filter, or for some reason your pickup pipe in the sump is blocked, dont use it till you get it sorted cause youll f@ck it big style, seems strange how its started since oil change, so i doubt its your oil pump but the 2 miles you drove will have given the oil pump a hard time now!!! bad how such a simple job can start a nightmare!!!, i would also have said to check oil pressure switch on filter housing, but the 'sounds like a bus bit' makes me think that its working ok and flashing as it should as you haveny got any oil goin round your engine!!!
i hope the garage is a good one and stump up for something it sounds like theyve caused!!
Gaz