Looks like using OpenDNS resolved this. Spamhaus test says:
{snip}
554 5.7.1 Service unavailable; Client host [192.203.178.107] blocked using zen.spamhaus.org;
http://www.spamhaus.org/SBL/sbl.lasso?query=SBL230
Terminating conversation
I raised the issue with my ISP but thaty haven't gotten back to me. My caching DNS server uses my ISP name servers to forward to so I'll need to see what's involved in going to OpenDNS assuming that they are going to be mega-reliable moving forward.