Reid's cancellation terms seem to be far more generous than many other retailers:
http://www.reidfurniture.com/acatalog/Terms___ Conditions.html
(I refer to 'cancellation', rather than to 'exchange', because it's likely that you'll have to cancel one contract and replace it with another).
The sooner you contact Reid's, the better your chance of getting things sorted out with very little financial loss (or, hopefully, none at all). If you're in Scotland, phone 0141 303 7400. If you're in Ireland, contact their Belfast office: 02890 783 700. (I strongly recommend phoning, rather than sending an e-mail, since it will avoid any potential delays).
If your order hasn't been processed yet, Reid's might well be happy to substitute your alternative choice of suite.
Chris