If it's a stock Bosch alternator with an internal regulator you can just change the regulator on its own by removing the 'lid' that the single push-on terminal sticks out through. Remove (loosen) the strap round the alternator then undo the three screws. The lid lifts off and brings the regulator with it. Two screws hold the reg. to the lid. That's all there is to it, apart from refitting it very carefully so you don't damage the brushes sticking out of the reg. I have Gen VW regs at £20 which includes P&P and it'll take you 10 mins to fit. Must be worth a try before going to all the hassle and expense of a new alt.