Use of home expense

I know this one has been done to death in general, but I have been thinking about a couple of situations for use of home expenses that will apply to some of my clients (and indeed myself). Apologies in advance  if the answers to the following are blatantly obvious and for the rambling nature of the questions!

I know we have the £2/£3/£4 (depending on which year) a week estimated allowance that can be used, but I'm thinking of when there are potentially higher costs that make it desirable to do a more full calculation. Specifically the question is what do you do when it is not just the client who pays the bills, e.g. what would you do for the following (assuming for all it is a sole trader or employed person working from home for a significant amount of time):

1. Two tenants each paying 50% of the rent, council tax and utilities, my instinct is that the expense can only mirror the actual costs directly incurred by the sole trader - so take 50% of the costs and use that with whatever apportionment is deemed reasonable (floor space/number or rooms etc), but the costs are "notional" in a sense as they are based on estimates and do not reflect cash physically being paid out by the business.

Also if you do assume that you only include 50% of the costs then can you reduce the proportion of the dwelling that you use to work out the business proportion? e.g. 6 rooms total - 1 used for business use, but one of the rooms is the bedroom of the other tenant - which is not really included in what your 50% of the rent covers, so that brings it down to 5 rooms? In essence I suppose the other tenant is subsidising the sole traders business if the rent etc is just split 50% which should in turn be included in the caclulations of the business expense.

2. Increasingly more common - sole trader (/employee) still lives with parents and pays (for example) £200 a month in board and does not specifically pay a proportion of mortgage/rent or utilities. I suppose in this case you would probably have to go for the £2/3/4 a week method as it would be very difficult to show any marginal cost the business causes to be included in that fixed amount. Additionally - what to do for the reverse - what if the client is recieving money in board, or from lodgers - should this be used to reduce the total costs for a dwelling before apportioning?

3. House is owned and all bills paid by a married couple (rather than some form of joint tentants/tentants in common like in 1.), bills may be paid from a shared account, or not and paid equally by both partners, or not. Let's say the rich wife (to avoid the traditonal stereotype!) contributes most of the money into a shared account from which all of the bills are paid for a large and luxurious house, the poor long suffering self employed husband also pays into the shared account, but not nearly as much, but he uses a fair bit of their (large and expensive) house for business use. Can he include all of the costs and apportion as if he was paying for all of it or should it be apportioned to take into account his contribution?

In essence this would be similar to 1. - the costs are being legitimately incurred by the business - but should the husband be able to claim tax relief when the wife has actually paid the bulk of the bills? I've had a look at the various BIMs - but they don't seem to look at this side of the equation - they start with the costs and then apportion them, when in actuality most people probably share costs in some way - whether as a married couple or some form of joint tenancy.