Thanks Rajshree, I managed to do it (sort of!). Unfortunately, I think I accidentally downloaded version 5.4 instead of 5.5. I used a link that had been posted on a forum and after I had downloaded I was asked something about downloading the 5.5 version. I didn't proceed because I thought it would register as a third download when I am only legally entitled to two downloads. Hopefully version 5.4 doesn't have much less than version 5.5.
I really hate this new method of purchasing software via download for this very reason. If your weak on the ICT side of things its easy to get confused and not really understand what you are doing. I purposely went out shopping to try and purchase this software in disc format but it was a waste of time because its just not possible to buy on disc any more.
Thanks for your advice.