Update from me. As I have reported already, I received my replacement lens last Thursday but unfortunately this too showed clear signs of being used. I didn't get a chance to email my "case officer" Matthew in the Customer Action Team until late Friday afternoon to express my "extreme disappointment" (among other things). I asked him to contact me so that this matter could be resolved directly without further delay as I had a function the following (now next) weekend which I had intended using the lens at. I copied in the CEO address. I got an email response Friday night to say that he was on holiday leave and that I should forward my message to
saveourcustomers@knowhow.com 
where it would be picked up and actioned by a colleague. I did that and got an email Saturday that someone else was on leave and then a further email on Sunday saying that someone would be in touch Monday. Phew, it was like pulling teeth again...
Melanie phoned on Monday and she was a breath of fresh air. Her opening line (after apologising on behalf of Dixons) was that she wanted to resolve this for me. She was conscious of the tight timescales as she was aware that I needed the lens by the end of the week. She said that they may not get this lens back in again (at all!) and with me being in Belfast another swap would not really be feasible in time anyway. She offered to try and locate a lens for me locally or if I wanted to buy one myself she would refund the difference. I enquired if there would be a discount available for keeping the used lens and she suggested that between 10% and 15% would be the norm. I took up the offer and opted for the 15%
.The refund was actioned there and then.
However, when I got home, I gave the lens a good workout comparing it shot-for-shot with my telezoom workhorse the 70-300mm F/4-5.6 IS and I just wasn't wowed by the comparison. Maybe I was expecting too much or it was just psychological with whatever seeds of doubt that had already been sown, but I preferred the results from the 70-300mm for most of the shots! Half way through, I realised that the 70-200mm was definitely back-focussing a bit - the DOF at 200mm/f4/1.2m is only a few mm so that wasn't kind to the comparison at the extreme closeup end, but even with that corrected with a -4 setting on my 7D micro-adjustment things didn't improve much to my eye. (I shot RAW and used a tripod, remote shutter and IS off BTW).
Anyway, I was originally happy (delighted!) to pay £752 for a new lens and now felt, rightly or wrongly, that the £113 saving on that for a "used" lens I wasn't sure about was a false economy, so I went back to Melanie today and she was happy to revive the original offer of me purchasing my own lens. So, as of lunchtime today I am the proud owner of a brand new (April 2011 according to the date code!) pristine lens from Jessops (as I waited in the queue, I did think of your post Pat and looked over my shoulder to see if I could spot the Dixons CEO
). I am leaving the old lens in to a local Currys store tomorrow and when that is confirmed, my refund will be processed.
So, it has been a long haul (nearly 4 weeks), but I now have a brand new lens for £752 (minus £32 quidco cashback - a £14 gain on Dixons!). A long haul but worth it, I think. I'm not sure I want to test this new lens now... (but if there are any immediate problems with it then at least I have a local store to deal with).
(I hope I haven't spooked any of you guys who have opted to keep your "handled" lenses - I'm sure the vast majority of them are fine, as mine probably was. If you're in any doubt you should be able to send it to Canon for a FOC calibration anytime during the 1yr warranty anyway).
