Liability on a 6 months rental

I am looking at a 6months rental, I own half of 2 properties in the uk, my X wife lives in one, we rent the other and share the money that's left over of any profit, do I need to inform any body in Spain of my arrangements or can I just come and have a 6 months holiday without any problems of the Spanish ax man, I will not be working in Spain I will live off my own money.

As long as you don't go over 6 months then that is fine. 

Thanks for that, what if did my 6 months, came back for a month, then did another 6 months is that a problem ?

If you come to Spain intending to stay 3 months permanently, or you stay for 3 month permanently,  then you will need to register on the EU Citizen’s Register.  As long as you do not stay 183 or more in a year you will not have an income tax obligation.

I very much doubt that in Spanish law there will be a legal qualification as to what sort of break one needs,  in order  to make a 6 months stay into  two  3 month stays.  That is, if  a day In Gibraltar, France, Portugal etc. breaks the period,  or if just a day of two are ignored when making the calculation. I pretty much guarantee that a lawyer would be unable to reliably say either,

Addition

Thanks for that, what if did my 6 months, came back for a month, then did another 6 months is that a problem ?

If you intended doing that from the start, you would be tax resident immediately.  If you just 'decided later' then after 183 days in total, you would then become tax resident and liable for tax on worldwide income, and even the income from letting in UK  (although taxable in UK, the rental income must be declared for any top-up tax which you might be due).