This question comes up frequently.
While you can check online that your vehicle is actually recorded as insured (
http://ownvehicle.askmid.com/ ) there's no way to find out the name of the insurer.
To do that you'll either need to check your bank (or credit card) statements to see if they show who you're making payments to, or complete this form
http://www.mib.org.uk/NR/rdonlyres/972F680B-7DD2-4C73-A889-2D61E4F3F927/0/DSAR_Form_1112_ISSUE.pdf
and post it, together with a £10 fee, to the address shown on it.
Alternatively, of course, you could simply remember where you put the important paperwork that your insurer (or broker) posted to you!