sql left function

Hi guys and girls

I am new to this particular forum so please be gentle
I need a logic to say whats the postcode in query1, and return the rm in query 2

however i if the rm_sales_band is between 2m-4m, ignore the postcode logic and apply the name TEST,

If the value is 4m+ then apply the postcode logic

Within query 2, pca there can either be 1character or two.

Eg pca

AB = Aberdeen
B = Birmingham


please find attached two screen prints
Query
query2

lookign forward to your help