You're now into the argument between a void policy and a voidable policy.
The difference between a void policy and a voidable policy is that a void policy is invalid and therefore would not cover that person using that vehicle on that road at that time.
A voidable policy is valid, but there is a defect in it which would enable the insurance company to invalidate if they wish.
However, it remains valid until the insurance company takes steps to withdraw it.
It's at this point that you have no insurance - not until.
An example of a voidable policy ( one which the company may withdraw ) is one as detailed above where unless the policy has a specific exclusion relating to non standard exhausts, it is currently a valid policy and the offence of no insurance is not committed.
An example of a void policy is the same rider using the bike for courier work where thereexists a specific exclusion in the wording of the policy excluding business or courier use - here the offence of no insurance is complete.
My view is that the circumstances we have discussed to date fall into the area of a voidable policy - the insurance co would be within their rights to cancel the policy, however they are under a duty to inform you in writing prior to doing so as it could be argued that you have withheld a material fact as regards the exhausts.
That's what I think anyway.....
I now have a headache as I last had to dredge this gem of legislation out of the depths in about 1992!
Andy