to a garage and they say I need a new radiator, but the sensor is not
on the radiator and I'm not loosing water, also the temperature gauge
never goes above half hot.

It's on the side of the expansion tank - facing forward.

Radiators are prone to going on 306's. I think that's most probably
your problem.

Stuart
http://www.radox.freeserve.co.uk/306.htm

Just to update this post. Ive been keeping an eye on the level and made sure that water is circling the engine properly (valve on the right hand side of the radiator and at the back of the engine bay) the light went off a few days ago and hasnt been on since. Ive done several hundred miles and its fine.
No changes have been made. Probably a faulty earth I recently had the power steering pump seal fail and covered the engine in oil. I still dont know where the Water Level Sensor is, no mention in the Haynes book. Although theyve changed it now to a "Service and Repair book" not as thorough as it used to be. The Temperature Sensor is mentioned, and this is on the block, I assume its part of this.