You still need to give them a chance to make good. If you don't, then any claim in the SCC under the SOGA will fail. I know exactly what you mean about not trusting them, but sadly you have little choice.
Special delivery will be fine, or even hand deliver it and get them to sign a bit of paper saying they've received it. Anything that gives you proof, basically.
That's a bit of a simplification. The OP needs to have exhausted all reasonable measures before taking it to SCC, but he does not have to give them the chance to re-do the work themselves. Just taking back your car and then summonsing them is unlikely to go well for you, but you don't have to give them a chance to re-do it themselves if you have good reason not to, which I think you do in this case. What you would still have to do is ask them to cover the cost of someone else rectifying their shoddy work, and if they say no, then you can issue a last letter before action, and then go to SCC if you feel the need.
DB