One presumes from your post that you were driving? What's the alleged speed and limit?
By not updating your V5C then you have committed a separate offence - the Police have complied with sending the notice(s) to your last known address. Essentially, your inaction has resulted in the requests going to the wrong address.
However, all is not lost.
Failing to update your V5C is rarely pursued but on the presumption you were driving then you can offer to plead guilty to the underlying offence (speeding) if they agree to drop the more serious failing to furnish offence.
If you plead guilty to the speeding offence then you can still go to trial for the FtF - you could end up with a minimum of 9 points and some nasty fines and costs.
You should plead not guilty to both offences (however weird that seems) but add the above offer. You may have to attend court to execute the 'plea bargain'.
