Steve
The first reply was I think deliberately madeto make it appear complicated maybe to encourage you to use the posters' business. In reality it is all quite simple.
When you move to Spain, as you will be intending to stay for longer than 3 months you will be required to register on the foreign citizens register (this was previously the residencia, that has been abolished)
Once registered you will automatically be tax resident in Spain and will be required to make your first tax return in the June in the year following your arrival. In Spain one declares tax for the previous year in the following June. So if you arrive this year you will make your tax return for 2012 in June 2013.
Having registered as above notify your tax office in the UK that you are now living permanently in Spain and are thus tax resident here. They will send you the paperwork to take to the Spanish tax office, who will return it to UK. You are then tax resident here..
You will be able to make your tax return either jointly of individually. There are allowances for individuals and for couples. Until last year it was advantageous for us to make a joint return, last year we did separate ones on the advice of our gestor.
If you are former UK government employee (police, army etc) then the pension is taxed in UK. Most other income world wide is taxed in Spain including private pensions and OAP. The gestor who completes your return will advise you
Ask around when you get here about which gestors to use. They vary a lot. We pay about 40 euros for two tax returns.