Hello,
the property makes about £18k in rent and the mortgage is about £13k and about £4k of that is interest. So we declare the rent income of about £8k each less some repairs and the interest portion of the mortgage. The issue I have is by the time we pay a combined tax bill of £5k the property is really operating at a loss.
It's not really operating at a loss (ignoring house price changes), you're paying off about 9k of the mortgage each year so your net assets will increase by that amount.
For example (per person):
Asset less mortgage at start of year: (100k asset, 50k mortgage) 50k
cash: 0
Total assets in business: 50k
Taxable income 7k
mortgage interest 2k
Tax on income 2.8k
mortgage interest tax relief 0.4k
capital repayments 4.5k
Asset less mortgage at end of year 54.5k (assume no price change, 100k asset, 45.5k mortgage)
Cash at end of year year: 7k-2k-2.8k+0.4k-4.5k = -1.9k
Total assets in business at end of year: 52.6
So you've made about 2.5k on an investment of 50k in this example, so a 5% return. But you've had to put money into the business to generate that, i.e. the business is cashflow negative, generating 4.5k of asset growth but needing 2k of cash injected to do that.
You might, of course have a lot less invested - in which case the return looks a lot better - or a lot more invested. I guessed a mortgage rate of about 5% and a 50% LTV.
One thing to think about is that once the mortgage is paid down the business should be cashflow positive. This can mean property is a nice way to use debt now to fund a pension later. I can't do the calculation in my head but I'd guess with these numbers you'd have the mortgage paid off in about 8 years and it would take another 3 years to repay the cash you've added to the business in the interim. (Do the calcs and then tell me how bad my guesses are

)
The numbers look better when only one person in the couple is a higher rate taxpayer as income can be shifted to the lower rate taxpayer but, of course, a couple with only one higher rate taxpayer isn't managing to take full advantage of the basic rate band of one of them. But maybe by the time you actually need an income from the property you'll be basic rate taxpayers.