I am Spanish and have come to the UK to work for a few years. While I am here my Spanish home is being rented out, and I put the income onto my UK tax return. My wife is not a UK taxpayer and I am a higher rate one. We own the property 85%-15% in my favour, but no form 17 has been filed, and so we report only 50% of the rental income on my UK tax return, which is to my advantage. But in Spain, I pay tax on 85% of the rental profit. Can I claim a deduction for the full amount of Spanish tax paid (subject to the usual restrictions) or would I claim 50/85ths of the Spanish tax on the basis that I am paying tax on a lower figure over here? 

If you had asked about your property income being reduced by an expense that is deductible under UK rules but not under Spanish rules, I would have told you to put the 35% figure in Box TC17 of the Foreign Tax Credit Relief Working Sheet, as explained in the Help Sheet 263. But in your situation, it is for a different reason that not all your property income is being charged to tax in the UK. I suggest you claim the full amount of Spanish tax paid, but explain yourself in the white space in the self-assessment tax return.