207 1.4 petrol, temp gauge and fan switch not working.

[Ianhw77k]

I've got a Peugeot 207 here that belongs to a friend. The temperature gauge goes to 70 when you start the engine but never gets any higher. We had the engine running, under load, for a good long time and it definitely got above 70 degrees. The fan never cuts in so it is in danger of over heating. Any ideas where to start looking?
I'm doing a service on it today so I'll have a bit more room under the bonnet for a while.
Thanks in advance.
Ian.