will manual DRS vmotion on ESX host failure

However if a host fails - what is the effect of this? - would the VMs move? - would I need to
follow a prompt to move - so if it was middle of night they  (VMS) would not be available?
esx 5.1

No, they would not move. If you do not have VMware HA configured, the VMs would be unavailable, and off, because the hosting server has failed. DRS only moves VMs using vMotion based on selection and loading on the Hosts.

VMware DRS is not responsible for moving VMs on host failure.

VMware HA if correctly configured, will restart VMs on another host should the hosting host fail.

VMware Distributed Resource Scheduler (DRS) Product Briefs

http://www.vmware.com/files/pdf/VMware-Distributed-Resource-Scheduler-DRS-DS-EN.pdf
http://www.vmware.com/pdf/vmware_drs_wp.pdf

VMware High Availability (HA) Product Briefs

http://www.vmware.com/pdf/ha_datasheet.pdf
http://www.vmware.com/files/pdf/VMware-High-Availability-DS-EN.pdf

DRS - it load balance the VM's when there is resource crunch in the cluster.

You would need to enable VMware HA for the VM's to failover when there is host failure in the cluster.

One add point is that services like DRS and HA use vMotion to move VM other host if need be. HA and DRS by these self can move VM's. As you can see most of the features in vCenter/ESXi are tightly integrated.

Thanks for the tip Hancock!!! ;-)

@comp no problems!

Thanks to all who posted