Thanks for the answer. Can I confirm the following please? You say:
"the best advice for this scenario is to perform a compression test on your engine and replace the plugs and let it finish burning off the carbon"
Is this something you would expect the garage who did the work to undertake at no extra cost? Is it a normal situation?
And one final question, how long would you expect is needed to burn off the carbon slag that was loosened by the cleaning? It's been driven for a 150 miles or so since the work (keeping revs as high as practicable) and it's still happening.
Thanks for the feedback.