Thanks Walter and Chidvi for your responses.
I have found that the problem was related to my MS Office user account. It seems when MS had a security issue a few weeks ago, I was automatically logged out of my account, and logged in under a different email address without me noticing. The MS Office programs continued to function normally despite no licence associated with the account I was logged in under. However when I was calling xlswrite through Matlab, Excel was behaving in a restricted manner. Now having logged into the correct account with appropriate licence, it works fine again. Cheers