Hi MartynThomas,
Could you check is Jambo Frame enabled on the virtual machine on the problem host?
I cannot re-produce the issue by beta driver, but I see some error:
2015-01-28T06:35:00.490Z cpu10:4273238)WARNING: elxnet: elxnet_dumpPkt:4892: P0 :: vmnic2-q0 Failure reason: "9k without TSO"
2015-01-28T06:35:00.490Z cpu10:4273238)WARNING: elxnet: elxnet_dumpPkt:4895: P0 :: pkt_len:11241, must_tso:0x0, tso_mss:0, num_frags: 4