I had a similar problem with the pistol grip on my Walther LG300XT Anatomical.
The previous owner could never fix it and neither could the UK dealer that sold it to him when new. It always had a slight movement and a slight creak that was both annoying and unnerving.
I had the chance to check-out another LG300XT Anatomical that did not have any movement in the grip and by changing the fixing bolt on mine I managed to cure it completely.
As you are local you would be welcome to come and take a look at it as I think you may have a similar "ball joint" type of mount on your LP400 as so we may be able to find a fix.
Failing that you could just leave it with me to save you any further annoyance
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.