Thanks everyone!
op hasnt come back
I hadn't come back to the thread before because I was enjoying the rest of my evening and er, sleeping!
Just to set things straight - I wouldn't pay her to do nothing while we were away, it has always been the case that she comes in to do the normal clean then any extra cleaning jobs that need to be done.
Looking down the thread the split seems to be even between those who think I am daft as a brush for even thinking about it
and those who feel the same and/or would have their cleaner come in to do the normal jobs while they are away.
Notes that some people even seem to be strangely 'wound up' about the fact that I might think this way! 
I take on board the argument about the nature of self employment and have freelanced at times myself. One of my family has been working as a locum doctor and of course, when they don't work, they don't get paid. But I think a self employed Dr, accountant, IT contractor or PR specialist is probably paid enough per day in order to set aside money to cover those fallow times. Maybe cleaners should ask for more to cover these periods, but what kind of bargaining power do they have in the market?
Also, the point about it depending on the original agreement is also a good one - 'We will need you for regular cleaning, but some weeks it might be more than others,' is a very different arrangement to 'We need you every Wednesday afternoon'.
But, clearly, the logic of 'she is self employed so should be being paid enough to cover those times' isn't panning out, as it has happened twice to me that these two separate individuals have run into financial trouble at this time of year - both giving the same reason.
It still just amazes me that it doesn't seem to enter these other employers' heads that although they are going away for two weeks, their cleaner might not be equally delighted to take a two week gap in pay. Find a property shows that the average price of a two bed flat in the local area is about £400K and a three bed house is £750K - so you have to be pulling in quite a big joint income to service a mortgage around here, yet when that couple goes off on holiday for a fortnight the cleaner working for £8 per hour should just go home and reflect upon the workings of a free market?
I don't feel remotely guilty about having a cleaner, but I also do feel a certain obligation to consider the effects that my decisions might have on her finances.