The 7series. Berkshire/Surrey to Heathrow, the easy way.
Map of the 7series routes.
70: Eton Wick-Eton-Slough-Datchet-Wraysbury-Heathrow Terminal 5
70A: Slough-Datchet-Wraysbury-Poyle-Heathrow Terminal 5
71: Slough-Windsor-Englefield Green-Egham-Staines-Heathrow Terminal 5
72: Bracknell-Ascot-Englefield Green-Egham-Staines-Heathrow Terminal 5-Heathrow Airport Central
73: Brooklands-Addlestone-Chertsey-Staines-Stanwell Moor-Heathrow T4-Hatton Cross-Heathrow T6
74: High Wycombe-Beaconsfield-Farnham-Slough-Langley-Heathrow North-Heathrow Terminal 6
X74: Oxford-High Wycombe-Farnham-Slough-Heathrow Terminal 5-Heathrow Airport Central
74 and X74 are run jointly between L1/Heathrow Buses, London OmniBus and Carousel. Both link Heathrow and Slough with High Wycombe
75: Maidenhead-Bath Road-Slough-Langley-Heathrow North-Heathrow Airport Central (24hr service)
24 hour service between Slough and Maidenhead, first and last buses are early and is very frequent. At every 10 minutes during the daytime, every 2 minutes early morning and late evening, and running throughout the night ensures you don't have very long to wait for your comfortable 75.
76: Britwell Shops-Farnham Rd-Slough-Langley-Heathrow North-Heathrow Airport Central
It also runs to Burnham, Garibaldi from Britwell on Sundays and evenings.