Help Please, I am locked out (Boot)

Hi all, I have just moved upto a 4.6 RR petrol from a 2.5 nice, but from day one the back door (boot) keeps locking it self. I went to SKS in Peterborough, and was told it was drivers door lock unit, so fitted new one, 5 days later back door is locking it self once more. we have changed the push button also. The back door (boot) just keeps locking it's self. Can anyone help please.

Graham