Technical Ducato MLGU6 gearbox 1st gear problem

[eiongibson]

Hi all, I am new to the forum so this is my first post looking for advice. I have a 2016 Ducato Multijet 130 panel van that I have converted into a motorhome. I have a problem with the gearbox. 1st gear does not engage properly and wants to jump out of gear. Additionally, when I hold it in gear it is noisy which is a worry. I have checked that the gear selection works fine, so something else seems to be causing it to pop out of gear. All the other gears are fine and I rarely need 1st gear but still would like to get it fixed as I suspect things will get worse and lead to bigger problems. Bit confusing as both 1st & 2nd gear use the same selector ring and yet 2nd gear is fine, can only thing it is a problem with 1st gear itself.
Has anyone else experienced this problem? Does anyone have access to detailed manuals on the gearbox, it is a 20GP18 variant of the 6 speed box.

Any advice would be greatly appreciated.