Thanks for the help.

To remove the OVP relay - you lift the rear most tab upwards and just as it clears its recess, push the relay aftwards so that the front tab clears its recess. There was a bit of resistance but jiggling and some persistance helped free the unit. I pulled the harness off fairly easily and then replaced the OVP relay with the reverse of the above. Nothing broken.

Removing the battery makes access substantially better - I can't see my hands fitting in there while the battery is in place. Also removing the battery probably makes for a safer replacement for any relay just in case it may be always hot (and apparently the OVP unit is).

Thanks again,
Paul