We are pensioners and have lived in Spain for a number of years.  During that time we have continued to pay UK tax as at one time we were told that we could as we came under a phrase “centre of vital interests” due to the fact that we own a property in the UK and our family are there.  I am increasingly concerned that this is not true and need help.  I am worried that if we go to the Hacienda they will throw the book at us and want back tax.

Our income consists of rental from the property and various pensions totalling approx 12k each.

Can anyone advise what we should do.

If you live in Spain full time as residents (ie more than 183 days per year) then you should be declaring for tax in Spain - even if you still have assets in the UK.
No the Spanish won’t throw the book at you - just go to a competent professional experienced advisor and they will be able to complete the forms to de register you in the UK ready to start paying in Spain. It is not as horrendous as it sounds - many have already done it.
Again - take a look here Pg8 for more information.
http://issuu.com/timadmin/docs/april12

First things first, I totally agree with the expatriator, they will not throw the book at you, and yes you need a competent tax adviser that can deal with both UK and Spanish Tax advice.  Try the guys as NeedAnAdviser.  Secondly, if you have been renting your property in UK and paying tax on it when you may not have had to, you are entitled to a refund of tax that was not due.  Any refund could then be offset against Spanish tax liability.  There are some forms that need completing in UK such as p85 (leaving UK) and NRL1 (UK non-resident landlords scheme ? to have without tax deduction in UK).  UK will then exchange tax forms with Spain and away you go.  It just may take some time as HMRC centre for non residents has a backlog of 16 weeks at the moment.  Finally, and just to be aware, Spain does have a new team of tax inspectors out there investigating expats and they are intent on tracking them down.  So, my advice is to get it sorted soon.
See http://www.expatadvice.com/html/article.php/type/article/zcid/1363/finance/Expat_Emigration_Leaving_the_UK