Just got given the above from a friend who had this as an upgrade but doesn't want it. I already have an old IPH5 which has been well used so the new one is preferable. Can I just transfer my SIM from my old IPH5 to the new one?
Yes, this is possible.
However, if you want to keep all your data etc, you will need to do a full backup of your old iPhone (e.g. to iTunes on your Mac/PC) first and then connect the new iPhone to do a restore from the old backup. This assumes, of course, that the new iPhone has at least the same capacity as the old one. If not, you may run into problems.
Also, make sure your friend has followed Apple's advice on What to do before you sell or give away your iPhone, iPad, or iPod touch.