Firstly Margaret, please accept my apologies for the long delay in responding. I had been out since first thing this morning, sorry.
I take your point, but we are now getting into rather grey areas. You cannot help but note the increasing acceptance of the legal system of your sort of arrangement. My answer was framed on the safe principle of splitting income in accordance with share and opening a door to extending that as has been done for many couples. I had grasped the position that you were tenants in common, indeed I mentioned it in my original reply.
Here is the opinion of Robinson Reed and Lawson, Chartered Accountants and Chartered Tax Advisers:
'As far as the rent from a property is concerned, the owners (whether joint or tenants in common) can agree between themselves how the income is to be split and the income tax on that rent will follow the split they agree on. It is important to realise that the rent must actually be split in the agreed proportions if this is to be effective for tax purposes.'
So you, indeed can, do as you wish. However I would caution you the have an written agreement in place in the event of an HMRC enquiry. My original answer was a definite 'belt and braces' position which should be cast iron and impervious to challenge.
I do hope that clears the air for you. Please be so kind as to rate me before you leave the Just Answer site.