I'm trying to lift the computer out to get at the battery but am unable to remove the plug.
I've raised the seat and removed the side panel and clip on plug cover then removed the tray cover so that I can see hole where you insert the screwdriver to operate the plug catch.
I've followed the instruction manual but with no luck, it just will not release the plug.
Anyone got any ideas?

Grahame wrote:
Anyone got any ideas?

Move it in the opposite direction; it will release if you're pulling the right side of the plug where the cord is attached while you're prying and muttering.

I just looked at the diagram in the 1994 Rider's Manual.   Not too helpful but interesting and could keep a person occupied for an afternoon.

The plug latch gets moved rearward. When you do that you'll be able to pull the plug outward from its right side then unhook its left side.

Thanks for the advice, it's done! 
I found a picture of the computer on ebay, once I knew how it worked it was easier but required a fairly firm push on the clip to free it off. Have now topped up the battery for the first time in 2 years.

Messing with the engine control unit and the battery hold down is why I am glad I have no-maintenance AGM batteries in my bricks.  Install and forget about them for 5-6 years.