"However, if I look at virgin terms and conditions when looking at either Economy (non flexible £425) or Premium Economy (non flexible £890)- It says I can make changes at a cost of £70 for either leg (Must stay a Sat night etc etc..)."
I suspect you will find that there is also a requirement that there are seats in the same fare bucket. If that is not the case (and with Orlando that feels a likelihood) you will have to pay not only the upgrade fee but also the additional fare to whichever fare bucket is available. The £70 doesn't guarantee you will be able to change it's simply a fee IF space is available.