Turned 29 and my car insurance cost has doubled through existing provider on renewal quote and Compare the Meerkat etc. Last year: £300. This year: £500-£600.

Has something happened in the insurance world that I’ve missed?

edit missed that you had tried for a renewal.

seriously though- try direct line. they have got real cheap of late.

both my cars are with them now.

Ask them to justify the increase. There must be a reason other than they think you’re a mug. After all, you have another year of NCB!
Try A- line (phone them) or as said above, Direct line.