Alko hitch

My ALKO 3004 hitch developed a knock and after some research on line the two most likely causes were the were worn friction pads or ensuring there was no movement in the overrun mechanism by applying grease to the two nipples underneath. So I set about tackling both problems.

Bought a grease gun/grease and greased up the two nipples underneath. The movement which was present before hand was eliminated after applying the grease and the over-run mechanism seemed to move in and out as required.

Sourced new friction pads both side ones and front/back and fitted them all. The front/rear pads were a spare set given to me when I bought the caravan new in 2012. When installing the rear pad I noticed that the new screw securing the pad in place was longer than the one I had just removed. Not thinking too much about it I duly fitted the new one.

After doing all this I hooked up the caravan to ensure it all coupled correctly. Although it was tight everything seemed to connect up ok. Unfortunately I didn’t take it for a test run as we were heading away in the caravan for a short break that weekend. I was confident that I had done both jobs correctly.

Unfortunately not long into the journey the caravan started knocking/creaking/groaning and did so all the way. When I got to the site the screw head had sheared off (?!) and was missing and there was a gouge around the tow ball ~ 10mm either side of the centre line.

Apologise for the length of the explanation but two pieces of advice/help/thoughts:-
- has anyone had a similar experience with the rear pad screw shearing off and more importantly how do I get the old screw out (as there is no head) to replace it with the original shorter screw?
- if the noises are not due to the two things I have done what are the possible other things that I should be checking?

Any help greatly appreciated as we are planning to go away again in a fortnight!!