electrical fault?

Hi,2009 fiesta style + that remote key fob stopped working. My mechanic transferred insides to a new cob but still only intermittently worked. Then car started locking and unlocking itself without fob being anywhere near it. Drained battery so took to main dealer. £108 plus vat fordiagnostic then 2 hours labour later they say key faulty and fault behind dashboard and it will cost another £500 as needs 4 hours labour.
Was considering just getting a new remote key and taking it to my own mechanic for dashboard investigation.
Any thoughts?
Thanks