Sam, from the description, it sounds like he's saying it's too small. Plus, he already said he got it from Halford's, not the LBS.

To the OP, I would think an 18" frame would be plenty big enough for your height, in most cases, so maybe the geometry is just not a good fit for you, and you should try a different bike altogether. Definitely try a larger frame at least. As far as whether or not they'll take it back, you'll just have to ask them.