I started this about 3-5 years ago
Lettings and one offs minimum charge £45 if it's a larger house , ie 4-5 bedroom detached its £65-£85 as I'm guessing they've not been cleaned in at least 6 months
Few Lettings agents I deal with wanted a call out charge on there books, So it's a £85 call out charge which they feel is reasonable , Obviously larger properties are priced higher
Never had any problems and get steady flow of one offs each month