[Laird Scooby]

It sounds very much like your alternator is putting out plenty of power (and to produce that it saps power from the engine, hence the lower idle speed etc) but also that the battery is either pulling too much current from the alternator or isn't providing enough of a back up power source as it should.

I'd suggest keeping an eye on the battery, try and find an autoelectrician to do a drop-test on the battery and advise accordingly - no gravity defying exercises are involved with this.
My suspicions in short are similar to Marks, your battery is weak and/or at the end of its useful life. Try to find a Halfords HB096 to replace it with, it's a traditional lead-acid battery without calcium or silver-calcium in it which need a much higher voltage to charge than your alternator puts out.
Also the HB096 is the heavy duty battery for yours so should have plenty of reserve power for starting etc.