I'm south of you, near Newton Abbot, and DIY was £15 a week ten years ago, that was with no bedding or farage, and no help. The most recently yard I was at (about 4 years ago) charged £14 pw, £4 ph for the school, and £1.50 to turnout each time, no bedding or forage.
If you're doing it for love, so be it, but don't get taken for a mug.
This is very cheap. I'm sure you don't need warning, but make sure you're not taken advantage of. These things creep up on you. People don't turn up, knowing you won't let the horse suffer. Or it's 'can you just see the vet/farrier when he comes?' Then they need a reminder of what DIY means. Still, I hope your customer is fair and really appreciates you. She should!!
£15 per week sounds correct to me; for pure DIY at your yard with the lack of basic facilities. However you are doing more than just offering them DIY so I think you should charge a little more for all of your time; I'd up it to £20 per week to be honest - they are still getting a good deal.