I have found a little more from Advice Now which rather changes the complexion of the situation. Here it is, it is about a boy friend moving in, but the principle is the same:
'I think it depends on how you will be living together. If you are sharing the home as a couple, then he is simply sharing the bills that you jointly incur. If it isn't 'income' you don't have to pay income tax. Once he's moved in, it will be your boyfriend's responsibility to pay the household bills and the council tax, just as much as it is yours - so he won't be paying this to you, he'll be paying the council, the electricity company, etc. The fact that the bills are only in one name does not change this, nor is it a problem if you want to keep the home in your name only.
If, on the other hand, your boyfriend will be a lodger, then what he pays to you would be income. However, you could use the rent-a-room scheme which allows you to have up to £4,250 a year rental income from a lodger tax-free. If you did not use the scheme, what your boyfriend pays would be income from which you could deduct expenses arising from the letting, such as heat and light, after which there might not be much or any profit left to tax. If you are going to do this however, your mortgage agreement will almost certainly require you to get the lender's permission before taking in a lodger and the lender might, say, increase the interest rate charged as a result. Your buildings insurance would also need to be amended to take account of any letting. Consequently, this would not seem to be your best option.
If you want to go with the first option, I suggest you make a living together agreement when he moves in. This will clarify exactly what his position is, what you'll each contribute towards the household expenses, and how that contribution will be treated. It is always a good idea as it stops problems cropping up in the future and you can do it in an evening.'
Phew: it looks as though you can slip under the Income Tax wire on this one fortunately. It took a lot of digging to unearth that opinion!