Hi Colin,
I am a sole trader and have been mobile for 18 years (don't think this law will stop me now).
I was working on a VW Sharan in the road just yesterday and this law did go through my mind, I just thought if someone wants to know what's going on i would just say "it's unsafe to drive due to brake failure". I think I might still get nicked because they would just say "recover it to a workshop".
Thankfully nearly all my work can be done off road so I may not have a problem.
I also think this law is intended to hit people that cause a nuisance, I think most people would object to someone repairing cars outside there house, day in and day out.
I on the other hand I turn up maybe once or twice in a year and make no mess, do not leave cars unattended on axle stands, have no problems with other neighbors (in fact I often get work from them aswell).
I do not see any possible harm I could be doing but I suppose there has got to be someone that would disagree.
Oh well must get on (off road today).
All the best, Kim.