Alternator not working

Been getting flat batteries in the bug lately. Found out the cause of this is the alternator not kicking out enough voltage. The alternator is a later one with internal voltage regulator.
When the engine is on its giving out 12.15v when the engine is off the readout is 12.25 (battery I guess).
Thing is it doesn't increase with the revs just stays a constant 12.15v which isn't enough to maintain the battery.
There is no batt light on the dash.
Anyone have any ideas what to do? I there a way of checking the voltage regulator or is it a case of just get a new one?
Not so keen on spending £100 if there's a simple fix.

I don't think you can do much with the internal units and you probably couldn't buy a new internal regulator anyway.
Just a thought are you sure the dash circuit is working okay if the bulb is gone I don't think the alternator will work, its not like a dynamo that will work regardless, the dash circuit acts as a sensor for the alternator to power up. I remember I had trouble when i changed over to an alternator and the dash ciruit was incorrectly wired up.
cheers
owen

Also you could try adding an earth to the top of the alternator body if you haven't one already.

Found the lead had come off the back of the battery light on the dash so got that fixed. It now illuminates then extinguishes after a few revs at startup. This should indicate its charging but I'll check the alternator output tomorrow when I have a multimeter to hand.
Really hope I dont have to purchase a new unit