Dr What?!

Are they trying to ruin Dr Who?
The 3rd series finished at the weekend and I thought it was the best one yet. David Tennant is a great Dr and I liked Freema Agyeman as the Dr's assistant Martha. I actually preferred her to Billie Piper (which some may say is sacrilege).
However at the end of the series Martha decided she didn't want to continue travelling in the Tardis so the scene was set for a new assistant for the 4th series. I was hoping for someone who would be as good as Martha but it seems that makers have gone for the worst possible replacement in Catherine Tate. Am I bovvered? Yes, very much so.
She is a dreadful choice. She made a guest appearance in the last xmas special and it ruined the show for me. Her character was just shouty, whiney and annoying. The whole way through you kept thinking that it was Catherine Tate, she just wasn't believable in the role. I can't understand why they have now given her the full time role of the assistant. At least Martha will be returning later in the series for a few episodes but I'm concerned that Dr Who may be going downhill now. It was one of the few consistently good home-grown programmes on TV, I hope that it continues but I have real doubts. I was looking forward to the 4th series but not any more.