Thanks again all.
There was a special plate - and yes, it is discontinued - as i have learned today. This plate had a channel to fit the 'scope mouting so it could not move left or right. Brilliant! So why discontinue it.
The answer may lie in the soon to be released Manfrotto "omni" mount which will fit all scopes - replacing specialist Leica, Zeiss Swarowski plates.
Good news? Well, the "old" plates retailed at arounfd £12 each - the "new" "omni" plate is...you've guessed it... £40 !!
My current plate has three hole, the centre one with the main screw and a second smaller one at the back which aligns with the second hole on the @scope mount - but I expect that the fixing screw is too short, so I will try to get a longer one.
Ideally I would secure in three places but am reluctant to let anyone drill into the 'scope mount!
The other alternative (other than to shell out £40 when the new plate is released) is to see if anyone out there has an unwanted Manfrotto/Leica discontinued but altogether useful plate?
PS - what does the Leica "mount" on the 'scope actually fit - it must be designed to fit something?