Thanks for all your help! I'll have to try all the suggestions one-by-one. Now to work through the questions:
fossyant, I am still going in an out every day, though I will sometimes have a midweek day off if I'm completely exhausted, it doesn't seem to help much! I have managed to shave about 20-25 minutes off my journey time since I started but again, that's not improved for a few months.
hackbike666, I'm 26. No idea how much my bike weighs!
Jonnyjeez, I ride a 21 gear city bike made by Holloway Cycles rather than a brand name. Don't have much of an idea about other specs! The route is from north London to south London, in pretty much a straight line, all main roads. There are no hills, but an incline for the last mile home that feels like Everest sometimes! I'm not very fit but I'm not unfit either. Been cycling for years though never these distances as I lived in a small city, I'd say I'm slightly above average fitness but wouldn't call myself fit.
ianrauk, I toggle between 9th and 12th gear.