I have not had issues with Jessops, but in general I know exactly what I want. I would dread to think what someone who needs advice lands up with.
In terms of good service though I can recommend highly ffordes:
https://secure.ffordes.com/index.htm
I live many hundreds of miles away from them but have purchased two second hand lenses from them and they arrived the next day and both in my opinion better than stated. I have spoke to them on the phone and there is no pressure they simply describe the condition and ask if you are interested, unfortunately Jessops cannot do this any more than Comet can advise you on HIFI.