You expect to "earn" your £10K back in 7 - 10 jobs?. per job. So you expect to CLEAR between £1000 and £1428 AFTER paying your 12% turnover fee and other overheads?. At the same time the helpful guy told you he did 27 jobs in February? So he CLEARED a minimum of £27K in a month?
!2% of turnover is massive, especially in a material intensive job such as boiler changing. Is the 12% pre or post VAT? Can you do your "own" work, without paying 12%?
If you are struggling to beat British Gas etc, given that you should be able to under price them, maybe something is wrong with your public persona, and you still have to sell yourself when converting the Homeserve leads. BG etc are difficult to beat price wise on their fixed price deals, but installs are a different matter.
I don't really get paying that much for, essentially, leads. How well are they qualified. How many leads did the guy have to service to get 27 hits. When did he get the time to survey, quote and convert. Does the £10K cover livery to your van, and stationery etc? Must the van be below a certain age?
I am not trying to just shoot you down, but these are just simple questions that popped straight into my head as I read your post(s)
Who "owns" the client? I am guessing that you do the install.and then they sell the servicing and repairs to another punter?
As Dan suggested, join the CC.
Oh, and ignore Dave Simms. I personally know a guy with the same name.
He is a complete knob as well.
Good luck.