Exactly the same thing happened to me today. After two days of running the fogs and occasional use of the Oxford hotgrips. I came out of Tesco on the way home today to find the battery too low to turn the bike over. Luckely i was only 1/2 a mile from home so i got a jump start battery and started it off that. I dont think it was a regulator fault as no warning lights came up on the dash. Have i got to keep the battery topped up every night with an Optimate?
Mark