So for everybody elses benefit with the help of one of my colleagues I did find the answer to this:
I added:
aggregate-address 222.222.222.0/24 summary-only
to the zebos config on the f5
Now when the DNS servers are working the /24 (instead of the /32) is advertised and when they stop working the /24 gets withdrawn.
The next stage that I need to to go to now is to add a second virtual server (within the same /24) also running a pair of DNS servers (for a different zone).
Then my logic needs to be if either of these virtual servers stop responding to DNS queries withdraw the whole /24. As the config stands at the moment both real_servers behind both virtual servers would need to go down before the route was withdrawn.
Any pointers are appreciated.