Just to update on things. Went back to the dealer who was V helpful and let me switch off all the display sets in the shop to listen to their display model, still a bit of hum but not nearly as noticeable, dealer offered to swap it for me but suggested I ring Toshiba helpline and get their take on it. Rang Toshiba spoke to helpful chap there who did confirm that there would be a background hum, but that he didnt think it should be so noticeable. Offered to get an engineer out but also pointed out that Toshiba have a 28 day swap policy (not sure if on LCD's only or extends to other products) and if I wanted a new one to go back to dealer. Now waiting for a new one to arrive, might be a bit of a wait though as dealer is waiting for more stock!
Thought I was going to have one of those "if its working then its not faulty" kind of conversations that I have experienced at Currys and Comet in the past.
Thumbs up to dealer and Toshiba helpline