The cheapest is probably to burn them to DVDs. A spindle of 25 costs �8.79 at Argos. (You can buy cheaper elsewhere but be wary of buying unbranded disks. Some are just as good as branded types but not all are). You'd get well over 100Gb of storage for that price.
thanks to both of you - I had considered the burn them to disc option, but was put off by the time it would take, definitely the cheapest option though.
I have just looked at the hundreds of external hard drives you can buy - do you simply plug it into the USB and copy the files over?