Any ideas as to why my temp gauge goes all the way up when I turn it off and goes back down when the ignition is turned back on?

The pre 2006 electrics & especially the dashboards are junk on the Scudo's & similar rebadged models, don't know about the post 06 vehicles.
Speedo's, temperature & fuel gauges all give trouble, check the engine temperature switch first but a pound to a penny it's most likely your temperature gauge is at fault.

That means changing the whole instrument cluster if that's the case. Will do thanks. Hope it's just the sensor.

You can get the dash reconditioned, don't know how much it costs though, maybe a 100 quid.
A decent 2nd hand unit might work out cheaper but then how long will it last?