Although weekly/monthly insurance is sometimes convenient, it's not always the best option and you won't see any great reduction in your premium from year to year.
I would suggest an annual policy which more often than not works out cheaper than what you are currently paying. Plus annual policies acrew no claims bonus, so if you go claim free for twelve months, you'll see your premium reduce. I would also suggest you contact a specialist taxi broker rather than a taxi booking office who offer weekly/monthly schemes. most specialist taxi brokers have a panel of insurers so they don't just have the one price.
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ChoiceQuote Specialist Insurance
http://www.choicequote.co.uk