Yes, Tiggie is right. They ignore the fact that you found the bike that was nicked, and the insurance never paid out. In their minds, you've claimed on one bike being written off and another being stolen, all in three years. It's a rubbish situation you're in, 100% not fair, but it is what it is.
There are companies out there that specialise in high risk insurance. If you haven't done so already you might try one or of these, but be prepared to open your wallet wide. They are usually ready to insure people mainstream companies won't give quotes to.
Other than that, you're only way forward is to wait until the write off from the oil spill expires: once 5 years are up, you no longer have to declare it, and you'll be back down to one claim. You'll be two years older too, which will help. But I can understand that's not what you want to hear if you've just a got a new bike you want to ride