Ok, recently became aware that charge light would only go out when bootfuls of revs applied. (Even then there was a very faint glow). Changed fan belt and checked tension - all ok. Wiggled leads around on back of alt. and generally had a look for any problem wires but couldn't see anything obvious. Next time on the road the charge light stayed out during normal driving over a 15 min period. After a short stop it was back on again. I bench charged battery and it was holding charge after a couple of days off the vehicle so guess battery ok. Checked the voltage accross battery with engine running and I was getting 14.43 volts. I've kept checking the battery over the last 5 days while the cars been standing and it's gone from 12.46v to12.42. I did a fair bit of mig welding to the dumb irons about a year ago (disconnected the battery) and a little welding to an exhaust hanger a few months ago. Is it possible that a fried diode would take this long to show a problem? If a diode is the problem how easy is to to replace, or is a new alt. the answer?

I'd pull it to bits and look to see if the brushes are worn. They are part number RTC3284 on a standard 16ACR alternator. Get yourself a parts manual - lots of the internals of the alternator are listed. I've never had one to bits so I'm not sure which bit does what.