Professional block management charges?

I know this is a hoary old subject, and the answers are usually related to the length of string, but I am just looking for some rough guidance in order to demonstrate value for money for our *self-managed* block. I have searched online and at a rough glance it seems that most professional managers charge either as a percentage of service charge (around 15pc?) or per flat (I have seen anywhere from £250 - £500 pa quoted). We are not necessarily looking to change, but in the unlikely event that anyone challenges my modest fees (two peppercorns and a peanut) I would like to be able to turn round and say "well if you had it done by the professionals it would cost you £x...".

Are these figures in line with your experience or knowledge? We are central London, 20 units, so on the basis of the above we would be paying say £5-10k a year. Does this sound right? Any information valued.

Thanks


A