Davie,
As this is an open forum, I’ll add what I did…
I put the SSR-100a inside the main enclosure, as I just couldn’t get it fit comfortably in the battery enclosure. I then wired the SSR trigger to a spare pair (brown pair) of the tether, and put a toggle switch in the top side tether interface box. Now I don’t have to get my butt out of the chair to turn the ROV on/off 
Even with the 300m BR tether, it works great. As the SSR draws so little current to fire, there doesn’t appear to have any impact (interference) on the data pair in the tether.
I bought one of those Thornwave relays, but haven’t replaced the SSR-100a with it yet. That’ll be for the next teardown/rebuild.
Cheers!
Ken