Never automatically renew with the same insurer.
Set aside an hour or two and get as many online quotes as possible and compare the cover and price.
Get the quotes from individual insurers and also "aggregator" broker sites like Chill, QuoteDevil, Bonkers, GetCover etc.
In my experience, very often a quote as a new customer with your existing insurer will beat the renewal premium - even after haggling with them to reduce it!
And in many cases going elsewhere will save money.
This year for example my AIG premium increased but I was able to significantly beat it by going via GetCover. (It was also by far the cheapest quote that I got all told).
If necessary try a broker or two - I normally don't bother but it may be merited in some specific cases.
Always double check that the policy details match your needs and don't ONLY shop on price alone.