Woohoo! I am a god! Or at least a very fit cyclist. I decided to give Windsor->Slough another go and despite leaving 5 minutes late, still arrived a couple of minutes before the train would have. I got to see it pull in :)
The train from Slough previous to the one I normally get (8:34) leaves at 8:24, so I'm not 100% sure I could get it. At least not unless I can shave another minute or so off the trip, which I could do as when I get to the roundabout before the A332, it all goes a bit pearshaped as I have to get onto the right hand pavement (there isn't a left hand pavement at the end).
*does a happy dance*