Dependant drop down list help

Hello All,

I am trying to use dependant drop downs to create an 'ordering' sheet and have encountered an error.

In the spreadsheet, column F and G seem to be working fine but then I have been unable to get an option for sizes in column H. This should be a drip down box so that people can select what size need but it should be dependant on what sizes are available for that item, eg boots come in shoe sizes and overalls come in chest size or M/L/XL etc depending on brand - I have been unable to create a drop down for this.

I also need column 'I' to automatically enter the item code, so for example someone has selected 'Arco ST202 Black High-Leg S7S Metatarsal Safety Boots' - this should populate the relative item code '6MM8904' from Sheet 3.

Any ideas please?

Here is link to the live test doc so you can see what I'm talking about:

https://1drv.ms/x/c/15153d41767146da/EYRmWmvKqGxEnwKisk8x4NMBl8lcNobK4Sz5RKb0UoLyjA?e=1IRk9Y

Many Thanks :)