I used to charge £10 /hr for anything (ride, muck out etc) but find people rarely want to pay that much for just groom / muck out / tack clean as there's usually a teenager in the area that will do it for 3.50/hr or similar!
£10/hr for the riding people have always been very happy with though.
Realistically prob no more than £7/hr for just muck out/groom/etc but guess it depends on your area.
... that doesn't look like a very helpful reply now i read it back to myself lol
