[QUOTE
I'm about to build a greenhouse on some existing paving slabs. The
slabs are very firmly laid, but are not perfectly level. I'm building
a 4' x 6' greenhouse and along the 6' length the slabs are virtually
level, but along the 4' length it drops by about 15mm.


[/quote]

You were lucky. Having struggled for 2 days to remove the old G/H and put the base together for the 8x10 we laid it on the ground and got the spirit level out :-(

With the back left corner on the ground the front right is 7" high, the front left 5" high and the back right 2" high.

This leaves me with a serious problem, if I dig out the back by 7" it means the gravel (the area is membraned with 2" of gravel on top) will be above the bottom of the glass. If I compromise say dig out 3" and thin down the gravel it still leaves a step at the door of about 9" which is silly and impossible to get a wheel barrow inside easily ( the base unit is 5" high)

Seems like a lot of landscaping is needed to level the whole area. :-( :-(