Just an update of what I finally settled on.
For my UK domains I am using Hurricane Electric recommended by billsimon. I was going to use it for all my domains but then I discovered changeip.com
While not free ($9/domain for DNS) I discovered that I could host my US (con, org etc) domains with them for $13/yr which is less than I am paying with a UK company and that would include full DNS maintenance. So it is basically free.
So I initially moved everything to HE but then started to move my com and org domains to changeip.
HE allows the TTL to be selected down to 5 minutes while changeip defaults to 30 seconds (as they primarily provide DDNS services) which is ideal for some of my domains, though 5 minutes would be acceptable.
They both provide DDNS services so I have also solved the issue I raised in an other thread
http://pbxinaflash.com/community/index.php?threads/sip-settings-external-ip.15359/ as to how to deal with having two external IPs. I have specified a sub-domain A record which is updated automatically by my router using the DDNS service if my primary line changes. I have then specified that sub-domain in PIAF's SIP settings as my Dynamic Host with a 60 second refresh.
Thanks to everyone for their assistance.