Result indeed - I'm the proud owner of said bike and very grateful to my clever husband for fixing it! Thanks from me also to everyone who helped on here, especially with the photos.
In fairness to the dealer I bought it from (Destination Triumph in Fareham), I should mention that they are in a 50 mph zone and can therefore probably be forgiven for not noticing this on a test drive. While we were diagnosing the problem, they said that I could put it to a local bike garage at their cost and have now agreed to supply me with a new Scorpion silencer (we agreed that the OEM one at around £600 wasn't really necessary).
Best wishes from sunny Shetland,
Birgit
I tried a scorpion on my XChallenge. You might find you need ear plugs!