I'm wondering about this as I have always just let meter readers in whenever they come. It bugs me that they just turn up and expect to be let in though. We have three different ones too.
I was reading a thread the other day about a bogus meter reader turning up at someone's house but they realised this when they tried to check his ID. I've never asked for ID or anything and I'm wondering if others do. I'm at home on Maternity leave with my 2 year old and new baby and it's started to worry me now.