Just an update. The only way we have been able to "Solve" this issue is to configure all hosts to use active/standby uplinks. The VMs then don't lose connectivity when they migrate. Not sure why but it appears the active/active options no longer work (from ESXi 5 onwards) we have tried both load based teaming and originating port id in the Active/Active configuration but neither seem to update the switches when a vm migrates. Both of these options worked previously. Something broke this in version 5. Now we only have half the available bandwidth being used by each host.
Also suprisingly in this instance it is not HP Virtual Connect at fault (a first!) as our IBM x3650 hosts are doing exactly the same thing.
Any feedback on why this is happening would be very much appreciated.