In our area someone charges around £7 a day, which includes, feed, TO and muck out, bring in and evening feed.
This person although only lives down the road, does keep her horses there too, so no extra traveling required.
Ahh so you aren't already at the yard? Well that does make a difference then. Perhaps you should charge your rate for doing the horses and then add on a certain amount of pennies per mile?
£20.00 is a very good rate, especially if you are reliable, and have high standards of horse care...imagine if you where on holiday and you got a call to say your horses are not done !! I pay 55.00 per week for diy...stable, hay, and two bales of shavings, full livery is £100.00
Are you doing a second journey to the yard to bring them in again? Or just one journey to turnout and muck out and someone else does them in the evening? Are owners away on hols (added responsibility) or just short of time?
When I was away last month I paid a mobile livery girl £7.50 per hour to do my two. She did not have to travel far though as she does horses at the yard next door.
£7 per day per horse does not seem out of the way to me if you are going them twice a day. That probably works out about the same rate as I was charged