I don't think any "guilty" parents are going to answer your questions, PF, because I think they are a rarity in the UK.
All the families that I know with au pairs treat them fairly and keep to the standard au pair working hours (25 hours per week, plus two nights babysitting, 2 full days off). "Sole charge" is when they get children to and from school, and for short periods of time - not for hours on end. The minimum au pair pocket money is in the region of £50 a week, but I don't know anyone who pays this low. I don't think that many au pairs would accept such a low payment.
I pay my au pair 80p an hour below minimum wage which basically means that she has room & full board for £20 a week - bargain. It's probably worth £100 a week if I had a lodger instead.
I believe that nannies - who do work long hours - get paid a reasonable sum for their duties. I have a friend with a nanny, and she costs them well over £20k a year for working 8 - 6pm.