Ancient Observer.. QF helicopters do not operate from Denham unless they visit there during a royal visit. I presume that QF still operate Wessex helicopters; if so, they are very bright red in colour. There are other fairly large helicopters, both civil and military, flying around the area where you live and into and out of Northolt. Specific routes are promulgated for helicopters, although twin-engined ones can fly off those routes just like any other aeroplane. The route which is close to you starts near Cookham, then directly to Iver, then south to West Drayton. The route passes roughly E-W just over a mile sputh of you. A helicopter seen heading south over your house would almost certainly be making an approach to Northolt similar to one conducted by a fixed wing aircraft. I.e. about a mile south of you it would turn on to final approach for the runway. The final approach in that direction has a "kink" in it to keep Northolt traffic clear of Heathrow traffic landing on runway 09L as the Northolt runway is some 30 degrees different from the Heathrow runways, the tracks converging to the west of Heathrow.
Hope that helps..