Given the lenght of time that you have been with your insurer, it appears a bit harsh, I would try and bargain the excess level down, I think that you are going to find it difficult to get terms from another insurer. Your current insurer is afraid that if 2 claims have arisen in a short period (2 Years) there could be an inherent fault with your plumbing system and is facing into more potential claims.
Bear in mind that there has been a savage price war between Insurers for Household Business and this is coming or has come to an end, Insurer have been losing money on Household business and the rock bottom premiums were unsustainable. I would also press the issue with your current insurer, that you have been with them 10 years and does loyalty not count for anything ?
Best of luck.