Thought I'd update this in case its of use to someone:
After opening a call with both VMware and Dell, 2-3 webex sessions and providing many, many logs to the support techies, it looks like its a problem with the Dell Equallogic SRA. Over the weekend I installed version 2.1 of the SRA (I don't think its been offically released yet!) and successfully added the array 
It's now (correctly) warning that the production and DR SRA's are different versions so I suppose I'll have to upgrade my DR installation (just waiting for Dell to confirm)