a lot of sales/lettings agents are a nightmare to get paid off. some however are very good. if you get foot in door make it very clear that you are setting a maximum credit of *** pounds. dont let them spin you any crap about having to wait til the rents come in. they will have money so simply demand it.
you will find that you cant command the kind of prices that lame suggests from lettings agents, they are a trade customer and will expect some kind of discount from normal rates based on the volume of work they are guaranteeing you.
round here (northwest) most agents will expect 35-40 for a cp12 and will be very reluctant to increase that if they are providing you with 200 a year to do. if you find their prices on the low side tell em its ok for one appliance only but properties with 2 apps incur extra tenner, 3 apps extra 20 etc, in my experience they will see this as fair extra work =extra money. also get approval to remedy minor faults for upto say 20-25 which allows you to seal flue, upsize condensate pipe etc
do not service as part of cp12, but make notes such as pilot burning yellow boiler in need of service @ £xx plus seals and gaskets @ £xx effectivey you are upselling on the cheap cp12 and overall it can work to be a tidy little earner. also note leaks from divertor etc, noisy fan....always send quotes in for this when you hand in the cp12s, as long as they know that it will be cheaper to fix now than if it packs in sunday 11pm they should approach the landlord and get you the go ahead for the additional works. note you cant do this on every property unless completely justified or they will soon come to think you are deliberately trying to rip em off.
it also helps your credibility if you do them the odd favour. i look after a large local agency and whenever they need something doing at any of the offices i only charge parts. this goodwill goes a long way. i also do the odd freebie as well, when a new tenant moves into the prperty and cant fathom out the controls/timer for economy seven etc i pop in when im passing and spend five mins with them explaining it. i dont bill for this as its not really out of my way and i earn about 30k turnover from them each year. again this generates goodwill.