Firstly, Thank you all for your help in trying to solve this. It turns out excel can quite happily accommodate more than 255 chars to a cell (presumable up to 32,767 as previously mentioned if you've got the ram for it.
So why didn't is work?
The short answer is I'm not sure. The first time I brought the data into excel I exported as a csv then opened in excel. This cut the columns short (at 255). (the csv file contained all the data when viewed through notepad but not when saved again through excel).
The column in question did contain some semicolons, but this didnt correspond to the cut off point (always 255) and I wasnt using ; as a delimiter.
My solution wast to do an export as an xls file. This has since been saved successfully as a csv file with the full data.
Hope this helps someone even though I'm still not sure exactly what the issue was here.