[wilbo]

Paul,

if you are using the standard strap that came with the vytec then I dont think you need to remove the computer unit from it at all.. underneath there is - what appears to be an oil filled cap. that is the entry point for the battery.

get a pair of large (or needlenose) pliers - open them up and use then to help you rotate the locking cap anti clock wise to unlock it.

the oring sits underneath the cap.

the battery should now be accessible.

put in new battery.
put in new oring - or reuse the old one if necessary

press down the cap - careful you don't knick the oring - and lock it by turning clockwise..