It should be there:
http://www.freeview.co.uk/Channels
If it's not, do a FULL retune. (If your Freeview box, or TV, asks you if you want to 'add' channels, or to 'replace' them, accept the latter option).
If you still can't get Channel 5 it means that your Freeview box (or TV) can't 'lock onto' the multiplex (= 'bundle of channels') which includes Channel 5. If you're receiving most other channels without any problems, a simple signal booster should fix the problem. Tesco & Asda will both sell you a cheap one (which will probably work) but Maplin will sell you a better one (which wil almost certainly work):
http://www.maplin.co....rial-amplifier-223504
Chris