Hi,
Have the garage confirmed its over heating also does it have this fault if you hold the engine at fast idle at around 4000 rpm? Does the temp gauge quickly move in to the red at high speeds and then quickly move down at low speeds then reason i ask is if the motor is over heating right in to the red it wont cool down that quickly if you just slow down as you have less air flow going through the rad.It may be possible that you have some sort of elec fault with the gauge or the sender unit.As for the rad when mine split at the top i was unable to get an aristo one so i fitted one from a gs,it will fit but the position of the top hose will cause it foul the induction pipe as it comes up to the throttle also the fan cowling mounting points will need a little modifying mine been in a few months and causes no problems as for cost i paid £120 for a new one.