Swift were pretty good in contacting us when we didn't send our proof of no claims off (I'd forgotten). Not only did I get about 3 emails they also sent a letter through the post saying my insurance would be cancelled if I didn't send it over to them.
EDIT: here are the NCB bits of the emails:
Email 1:
If you have earned a No Claims Discount, you must send us an original proof as soon as you can to swiftcover.com, Swift Court, Fudan Way, Stockton-On-Tees, TS17 6EN (your last renewal quote will be fine, as long as it shows your NCD entitlement - please write your Swiftcover policy number at the top of it).
Email 2:
Dear xxxx
Thank you once again for clicking on swiftcover.com for your car insurance.
This is just a swift reminder to let us see your original proof of No Claims Discount (your invitation to renew document from your previous insurer will be fine, as long as it shows your NCD entitlement). Please do not send us your original Certificate of Motor Insurance as you may need this for future reference.
Email 3:
Dear xxx
We note from our records that your proof of No Claims Discount has not been received. As it is a condition of your swiftcover.com policy, please send your proof as soon as possible.
The original renewal notice from your previous insurer will be acceptable, as long as it shows your NCD entitlement. Please do not send us your original Certificate of Motor Insurance as you may need this for future reference. The proof must be no more than 12 months old, issued by a UK insurer, it must display your name as the policyholder and the NCD should be earned on a private car.
Should we not receive your proof of NCD by 07/03/2008, we will unfortunately have to reduce your NCD to zero years or, in some circumstances, even cancel your policy. Please send your proof of NCD by email or just pop it in the post with your policy number at the top and send it to swiftcover.com, Swift Court, Fudan Way, Stockton-On-Tees, TS17 6EN.
Pretty unlucky that you didn't get any emails (I'm sure i would have got more if i hadn't sent it off before they cancelled it), and the letter must have got lost in the post too!