I did this when I first started up, was on £21 + VAT for "service", did three days a week, always got 12 services a day in the same area. They binned me off after 20 months which wasn't a drama as I was getting established by then.
They then reapproached a few of us a year later, gave us an additional £1500 if we committed to three days a week so I went back (whilst doing my normal work in the remaining 3 days a week-kept all my existing customer served as I'd seen their loyalty towards contractors).
After a couple more years they started introducing breakdowns into the mix, which would involve a recall if parts were required and this is included in your breakdown fee of £55 + VAT and would then spill over into an "off" day. This coupled with the fact that it was STILL £21 a service despite 5 years having passed, we were being QC'd with phone calls direct to the customer to ensure we had discussed offering them energy from BG, a new boiler from BG, can we sell you a Magnabooster etc. I had assumed I was there to provide a service, not be a saleman.
A good firm to cut your teeth with as a contractor, was invited to the curries out with the regulars (until their union got involved) but you will get talked to like a second-class citizen in the team training talks by some of the regulars.....