"Muesli or porridge for breakfast"
+1 to this.
I tend to find that plenty of excercise the day before leads to your body repairing itself overnight, which in turn means that by the next morning you are seriously lacking in blood sugar - and this manifests itself as the munchies by the time you sit at your desk. I used to get exactly the same thing, and have found that a big bowl of porridge with a teaspoon of honey keeps me happy until lunch.
I really CANNOT understate the effectiveness of porridge in this situation!
If you are concerned about putting on weight, can I suggest a body fat monitor (about £5 - £10 from a chemist), as you may note that as your muscle builds your core fat (the really nasty stuff sticking to your organs) drops, but no weight is lost.
Remember that it's not just about what the scales say - it's about how you feel about yourself, and cycling only affects that one way...
(Although if you ABSOLUTELY want to see the scales move, the biggest difference you can make is to cut out the booze. Sorry.)