Oh dear wrong again, becoming a habit this false information,
"Under the 2006 law a plumber is allowed to carry out the basic task of fitting such item as a water tank emersion heater during the course of checking a fault in the hot water system, the law recognises that it's impractical to the house owner to have to have two trades persons to do this minor connected work"...
If they choose not to that is entirely up to them.
With the MOT law the tester does not have to be a mechanic, this person only has to prove he has the nessary knowledge to inspect and check over any car due for its MOT, this person has to pass the inspection test before they are allowed to carry out said work.
I got some compensation and an undertaking that although I would do all the services, to ensure they were done correctly, the guarantee would still be valid....This highlighted is your post which is also untrue and false.
Back when you had this car the guarantee was about one year, some did two, not many, no garage could legally tell you the guarantee would still be valid because the guarantee for any warranty work was not with them but the maker of the car, say a Ford, the garage would repair the car for free under the terms and conditions then send the invoice to the maker, who then decided if the work done was indeed under their terms, if not it got bounced back to the garage to pay it.
Your guarantee was not valid just because you told them otherwise or they told you, they probably only told you this to get rid of you.
You could never ensure, or convince any garage any work or services that were done by you were repaired correctly and legally because you were not qualified in this field of work, or employed by the garage, if you did repair the car and had no major problems you won, had you had a major problem which required the garage to fix it, and it was found to be due to some bad servicing you had done, then you were on your own and the garage was legally entitled to tell you to go away, it matters not how careful you drive, even you have no power over car parts wearing out.
It's only in the past years that garages cannot insist that you have the suppling dealer service the car to keep up the warranty, but you might find it still has to be done by a registered garage who is VAT registered and not some "Fred in the shed' back yard mechanic.