Hello, I am Keith one of the experts on Just Answer and happy to help you with your question.
You most certainly are liable to Spanish Income Tax. Here is a very useful summary from AngloInfo:
'You will become liable for tax as a resident of Spain if:
In Spain, an individual is either resident or not resident for the whole tax year. Part-year tax residency is not part of the Spanish system.
As As a resident of Spain you will be liable for tax on your worldwide income at scale rates after any available allowances and deductions'
So there you are, simple, as the Meerkat in the TV advert would say.
All is not lost though as through the Double Taxation Convention between the UK and Spain any tax deducted by the UK is allowed as a tax credit against any Spanish tax liability on the same income stream so you are not taxed twice. However, the Convention does not protect you from differences in rates of taxation.
I do hope that this answer has been useful to you. You may well have to employ a local agent in Spain in this matter.
If I declare my UK Taxes to the Spanish Tax Man, can he ask me to submit subsequent Tax Returns for up to how many years?