I have tried, so far with no success, to glean some information about income tax levels on State pensions. A friend though the rate was 20%. Sounds about right. Any comments appreciated.

This is not as simple as it may seem. The source of the pension, type of annuity, etc etc all can affect whether you are liable for tax in Spain or the UK - fortunately there is a double taxation treaty so you won't have to pay both.

I am not an expert but I do know enough to say that you should not 'wing it'

You must get advice from a PROPER pensions/tax adviser - one in the uk that is FSA regulated.