Mr Green wrote:
I am reseating a greenhouse on a concrete base - about 8ft x 6ft.

I will seat it on tanilized timber lats of say half inch x one and a
half inches - with rawl-bolts in the base - through the lats - and
then through the green-house base.

Why?

The base is not perfectly level - once side appears to be about half
an inch on the low side wrt the other three sides

I am minded to lay the wooden base lats on a mortar mix - just a skim
on three sides and then half-inch on the fourth.

Any points? - use a mortar mix or some other pre-prepared mastic or
similar - or even make as level as possible with mortar - let it go
off - and then lay all timber on mastic?

Why not let it go off, and then fix the greenhouse directly to it?

I intend to buy the tanilized timber lats - would you treat them any
more before use?

You are creating a problem which is unnecesary - the timber will rot -
especially if it's only half an inch thick, and within a few years it will
be gone, leaving gaps which the GH will slip into...simply use bolts or
screws direct into the concrete if you wish to fix it down, although it can
never blow away once glazed anyway.