Ok, I've decided to approach this slowly, so rather than launching a big all out extreme new exercise regime, I'm going to make little changes.
My first little change is that I will no longer walk up/down my home staircase once... I will go back and walk up/down it 3 times.
Now I'm not sure how much difference this will make, but I looked at the maths. Judging by the figure I found this will burn an extra 10 calories everytime I do this, over the course of the day I must take those stairs at least 10 times, so that's an extra 100 calories a day gone.