Don't touch a lawyer unless you are really in the s**t as they will double your costs. Example if you owe 4 years worth of tax that should run up a bill of around 600 euros for a typical apartment on CDA. The lawyer will add another 600 euro at least for sorting it out!! Whereas you should only pay £25 per outstanding tax year via Spanish tax forms.
If you are not registered with the Spanish Tax office, that may complicate things a little so advice first is the best way forward.
Try http://spanishtaxforms.co.uk/ or enquiries@spanishtaxforms.co.uk or give them a ring for advice first.