A check of this flight on flightradar24 shows that the inbound aircraft was late departing RTM this morning (http://www.flightradar24.com/data/airplanes/g-lcye/).

It was also late departing LCY on Sunday night, which probably meant it missed a servicing slot at RTM, which meant a delayed Monday morning start. That's my guess, anyway. Others with more knowledge of these things will no doubt be along forthwith to stamp on any flaws in my analysis/guesswork.