DDNS not removing records

I know this is a little specific but I'm hoping some DNS guru might be able to help me :)

I'm running bind9 and isc-dhcp-server on a raspberry pi to handle local dns for my home network. I've set this up many times before on ubuntu virtual servers with no issues. I have configured DDNS and notice new records appear with the correct TTL of 5 mins in my zone files. However, they don't get removed automatically after their TTL expires. I noticed I still have records from workstations I renamed weeks ago.

I used the excellent guides from Lani's blog that has never steered me wrong before:

http://lani78.wordpress.com/2012/07/22/setting-up-a-dns-for-the-local-network-on-the-ubuntu-12-04-precise-pangolin-server/

Since doing this on the raspberry pi I've run into odd little issues. I had to also set 'dnssec-validation no;' as I couldnt resolve some websites, and this seemed to do the job. I have also manually installed the resolvconf package as this isn't present by default in the Pi.

If anyone has any ideas I'd be very grateful.