Ok, Having done a bit of panicky research across the forums, the suggestion was to disconnect the battery for 15 mins and hope for a reset - which I duly did.
This appears to have cured the issue with the HEC as all appears to be functioning correctly. The SRM module also still works so feeling like a very lucky boy at the moment.
In light of my antics this morning my best advice would be to STAY AWAY FROM HEC CONFIGURATION PAGE 3 unless you know what you are doing.
Thanks all for your help