time you spend at the gym depends on what your working, hour a day could only properly cover 1 body part, as i understood it. eat something after an hour or so and you could easily do 2 hours to do multiple parts, but then your limited on what you can train the next day. 2 hours everyday would be over training and defect the object.
80kg is a fair amount for someone weighing 12 stone id say.
you could sneak a snack in through the day to keep your protein intake between breakfast, lunch and dinner while your working, but going to the gym in mid afternoon isn't a must. your pain barrier actually increases in the evening, to for weights this is a recommended time to train over afternoon sessions (from what ive read), this obviously depends on how physical your job is.