I contracted direct with bg a year or so ago and the rate then was £18 a service and £50 a completed breakdown if you use there van and slightly more using your own van but then you had to pay for diesel,i enjoyed it and you can earn decent money as the people that pay through the nose for there cover dont tend to do anything so repressuring combi's and changing batterys in thermostats is common for £50 in the winter on a cold day you will earn loads with frozen condensates,down side full strip clean on back boilers for £18 can get tiresome.
They will expect you to sell like crazy co detector,spirotech's and arrange boiler leads for boilers where they state parts are not available.
An average day would be 8-10 service or 7 or 8 breakdown with selling commision on top.
I left because i wanted to do more varied work ie installations as well but a few of the lads i was with got took on full time,
if your not really busy give it a go you've nothing to loose.