rover 75 overheating?

hi i have a rover 75 on an 52 plate 1.8 engine.
was driving home from work today and noticed the engine was overheating the needle was pointing straight up pulled over to allow the engine to cool .
drove off 15 min later and the temp shot back up .got the car home and noticed water all over the resevoir on top of the rad.left the car to cool for 3 hours then topped up the resevoir and drove car around the block and the temps went shooting back up again but the water in the resevoir was still cold,please can anyone help with a solution as i have just paid £650 for a whole new clutch system and had the head done last year and i dont want to get rid of this car .