Problem with tow ball?

You do certainly need an inch or two for the hitch to sit on the ball.

7mm between the bumper and the ball doesn't seem to be enough space for any type of hitch so I would get the vehicle dealer to sort it!

The operation of any motor mover is not precise and a bit of overrun in the last moment before landing exactly above the ball is to be anticipated.  I think you have cause for concern.

Agreed it seems a narrow gap to play with but you can add a steel bumper protector plate onto the tow bar.

http://www.towsure.com/towbar-bumper-protector-plate

 Designed to protect against accidental damage...

Agreed it seems a narrow gap to play with but you can add a steel bumper protector plate onto the tow bar.

http://www.towsure.com/towbar-bumper-protector-plate

 Designed to protect against accidental damage...

OP has a detachable towbar 

If Westfailia say you only need 67/68mm and you have 75mm then that leaves you quarter of an inch clearance.

I disagree with both comments above and think you'll be fine. My motor mover is capable of enough accuracy to hitch up without fouling the back of my car and I'd guess there's similar clearance ..... I'll measure it for you when I get 5 mins

In order to reduce stresses in the components, the towbar manufacturers try to keep the towbar arm as short as possible. 75mm from centre of ball to the bumper is quite normal and there is no way of increasing this dimension as the towbar has been type approved as it is and any modification would make the approval null and void (unless the manufacturer has provided for a longer arm in his documentation, but that is unlikely).

same post on two different sections and with different answers. Best to not double post

I would go with Lutz as the gap sounds reasonable but if the clearance proves insufficient then it is not fit for purpose and you can ask for your money back and find another make of towbar. However I cannot see that Westfalia have not done their homework and got the gap right.

Thanks to you all for your help and advice its much appreciated.  I will let you know how it goes when I collect my caravan.  Will probably have another concern for you to help me with before then.  Once again many thanks