Hi Kara, we have the same bed as you and we have the Mothercare cot! I got it for £35 in eBay though, wasn't going to pay £200 for it. I hadn't seen the reviews till now. We haven't yet tried putting the fourth side up so haven't had any issues with the clips.
What we have done is the cot is up against the wall, and then the bed right up against the cot. We have covered the hard wooden edge of the bed with a towel and then a sheet over that. It does mean that there is a harder bit of surface between the two mattresses but it hasn't caused any problems. We have the cot mattress on the lowest setting but it was still too high to be flush with the mattress, so we simply sawed 10cm off each leg of the cot to make it the right height.
Hope this helps. I would advise getting a full size co-sleeper cot (or even a normal cot and altering it) rather than the little armsreach type, as they are very expensive and you won't get all that much use from it before the baby outgrows it. We're still happily using ours as a co-sleeper cot and DS is 9 months; hoping to sort out his room so that we can move him in there sometime soon and then we'll just use it as a normal cot.