I've had a Carole Nash Custom policy with Equity Red Star for 5 x years (originally covering my "modern" 612cc Royal Enfield Bullet GT Cafe-Racer, and subsequently a "classic" Moto Guzzi, a Ducati Monster 900, & recently a Ducati ST4 as well) and previously had to list all modifications to the Bullet on a separate form & provide pictures of it for their underwriters.
Calling CN to renew my policy this week, I mentioned the non-E-marked (Viper) cans on the Monster, and they said "the bike is not covered, and any claim would have been thrown out immediately". I pointed out the Bullet has a "Goldstar type" silencer that I had listed years ago, and they said in that case if it wasn't E-marked (it's not) then the Bullet also wasn't ever covered! They said I should have told them about any non-E-marked exhaust, although they never specifically asked me about this. (The 1986-imported Guzzi is borderline for needing an E-marked exhaust, and has Lanfranconis that are stamped "competition use only" - in Italian which my local MOT tester doesn't understand!)
All my silencers have passed recent MOTs with no problem, but Equity Red Star via Carole Nash are only happy with E-markings. However, via other brokers (e.g. Ducati Insurance) Equity WILL cover a bike with non-E-marked exhausts (albeit they'll only pay for replacement standard ones), so it is clearly NOT a legal or underwriting issue that's at stake. This is a common modification so you might expect CN to pro-actively ask the question before insuring any bike: but they don't! Why not?
I suspect that Equity probably gives CN a rock-bottom deal, enabling CN to offer their customers a good price + make a good profit: in the event of a claim on a bike with a non-E-marked exhaust, Equity will still have to pay out any liability to a third-party, but will then reject the policyholder's claim for damage to their own bike - thus significantly cutting their claim costs.
So, if you tell Carole Nash about your non-E-marked exhaust, they will most likely offer you an alternative, costlier policy with another insurer. If you don't tell them, then in the event of a claim on your own policy you've actually only got Third-Party cover, not comprehensive. Is this a scam or what?