While it's possible to check online whether a vehicle is actually insured or not, the name of the insurer isn't available online.
If you've forgotten who your insurer is (and you can't find those important documents that you were meant to keep in a safe place!) then you only have two options:
Firstly you can look through your bank statements to see if you can find out who you're paying your premium to or . . .
. . . you can complete this form
http://www.mib.org.uk/NR/rdonlyres/972F680B-7DD2-4C73-A889-2D61E4F3F927/0/DSAR_Form_1112_ISSUE.pdf
and send it with payment of £10 to the address shown at the foot of it.