You have lived in Spain for 6 years, your lawyer told you that you did not need to pay taxes, when you bought your house steuererklärung "because you get a pension from the UK". Wrong!!
The Spanish Tax Laws, which have not changed, state CATEGORICALLY that if you spend more than 183 days per calendar year in Spain, you have to register in the Spanish Tax system. "Double taxation agreement" does not mean that you can CHOOSE where you pay your taxes.
There are exceptions, for example if you have worked for the British Government as a civil servant or member of the armed services, you can only be taxed in the UK on that income. If you get a state pension and other private pensions from occupations you may have had after leaving government service,then those incomes must be declared and taxed in Spain
Failure to register can result, not only in fines for non-submission of tax declarations, but the tax itself. The Spanish Tax Authorities presume that if you haven't registered, you are "non-resident" therefore should be paying "wealth tax" (which we believe was abolished in January 2008, though you will still have to pay it for 2007).