I travel daily from High Barnet to London Bridge (or did when the
trains still ran through the city branch from Barnet...) The daily
single fare is £3.40 (although I have an annual travelcard)

Following the Camden derailment I made a claim against London
Underground. I have just received my refund... of £46. However, the
covering letter states that the refund is based on the single fare
multiplied by 20 (ie. 10 weekdays worth of lack of service). Now, my
arithmetic would suggest I was due £68 rather than £48. A colleague of
mine who does the same journey each day made a completely independent
claim for the same period and was refunded the same amount as I was.

Does anyone else have experience of this, or know why it's happening?
I feel a letter coming on...