I need to make a concrete base 5 foot long by 3 foot wide and about 2 or 3 inches thick. Can i use sand cement mortar for this or will i need to buy the proper path cement that they sell in b&q.
you may wish to go 3-4 inches or 4-5 using stone filler base.and a angle for drainage.if you drop a 25lb bag of soil on a stack of 2 or 3 you might crack your fine work.
We laid garden slabs for our shed which was much easier - they are still fine although the shed is rotting now, some 30 years later.
Would slabs be another option for you?
Ballast/gravel mix with cement is the only answer for a concrete base. Have you thought about buying cheap slabs, providing they are laid correctly they will do just as well.