Obviously your insurer (Endsleigh) would try avoid their legal obligation to accept the claim
Unfortunately, your occupation rules out the chance of saying how was I to know that it had an air filter and different exhaust to the 'manufacturers standard specification'.
Its a difficult one, you could go down the lines of ignorance or maybe cash in hand to the 3rd party - although thats risky because he could claim aswell as take the money.
Insurance will be harder to get + more expensive for a while to come
