my beloved 17 40 seems to have some problems:(
  A while ago I  accidentally knocked  my lens on the door while  the camera was hanging from my shoulder with the strap. just after that when I tried to focus  it didn't work  I un-mounted it and mounted it back  then it seemed to be working  but   after using the lens yesterday I noticed all the images came out blurry .I did a quick test at home using a tripod  and the results were absolutely disappointing.   any ideas what might have  happened and I would like to get it checked but unfortunately it is no longer under warranty. I was wondering if you have  any suggestion on where I should get it fixed in London.   in some forums they recommend  lehmanns and fixation  does any of you have any experience with them? or would it be wiser to send it to a canon repair service?  I hope it is not going to be too expensive to get it done.

thank you very much for your responses in advance?

Thank you very much Richard for your response. Your help is much appreciated as always. Thank you!