Hi Nath,
Yes your right there was no spaces it was part of the casting with a line in the casting which I thought was a shim. It is the 5 speed, I marked the casting with a gauge and took 3mm of the part of the casting that I thought was a shim (it is threaded all the way up) and mounted the gearbox again. It worked really well with the pulley no longer catching the underside of the gearbox mounting plate. I started cutting the grass and it was working much better than before, but then it stopped again.
You are right about the circlips either side of the pulley and there is also a small washer on the side of the shaft when you take the pulley off, which must be a small spacer.
I think the pulley is catching again because it is very hot, so there must be a problem further up the shaft, but not sure how the pulley is moving upwards with that circlip on the shaft above the pulley so the all shaft must be moving upwards, which must mean some part is worn out within the gearbox.
Sorry I do not have any pics (I took some but cannot find where they are on my phone) the parts diagram shows what I am talking about quite well. PDF on this page 3rd up from the bottom
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=7306#Post7306
