Hmmm... I had no formal training and was pretty much self taught. That said, I used to work in a garage, as an MOT Tester (Obviously I had to do the relevant exams and qualifications for this

) and mechanic. I had no issues, bar one which was a customer going against our advice. Essentially having a head skimmed and head gasget replaced on a Vauxhall 1.7TD (Izuzu) engine, even after being told it was a waste of money and a NEW head was required! The customer was forced to sign a disclaimer before any work was carried out because we knew it was not going to work... They were a bulletproof engine but the heads could not be skimmed, simple as!
We were also a Green Flag agent (amongst the others but GF we were the sole agent) in the Basingstoke/Alton/Tadley area and my roadside fix rate was 97%
I accept Markey's comments but feel it was a little extreme. I too have spent many hours repairing a bodged DIY job but paperwork doesn't mean everything... I spent hours repairing "professionals" work also. It's down to experience

I also agree with James... Insurance is important. If I asked someone to do a Cambelt, for example, on my car (paid or not) and it went wrong I would want some way of recouping the cost of repairing/replacing the engine and I, personally, am not the type of person who would then go back and demand it was covered by them. If they had liability insurance and offered the chance of getting the remedial work done without it costing them an arm and leg I'd be more than willing to do it.
TBH I'm in a lucky position in that I am capable and willing to do most jobs myself but I (normally) only help others by looking in and advising

As several of us have said before, I am more than capable of doing a LPG conversion for someone but I wouldn't take the job on paid, I'd just help them do it... They would have to get involved

Just my 2p