Brilliant. Just tried this, and specifically enabling 'Forged Transmits' fixed it. That step wasn't on any of the guides i was following. It seems that ESXi 5.0 had these policy settings set to reject/accept/accept whereas in 5.1 these settings are set to reject/reject/reject, hence Forged Transmits was allowed by default previously.
Thankyou *very* much 
Here's a couple of relevant links that mention this requirement for further reference.
How The VMware Forged Transmits Security Policy Works via @ChrisWahl | Wahl Network