This is one of those tricks that, once you've seen it, you wonder why you never thought of it yourself. All you need is a hosepipe that can stretch all the way from where you want to
take a level to where you want to
put a level, ie; from the old patio to where the new one will be.
Find some way of supporting one end of the hose in an upright position at the point on the old patio from where you want to take a level. It's easiest to do this by driving in a wooden peg, stake or garden cane at the edge of the patio, and having 450mm or so of upstand. Use twine or string to lash the hose to the support stake so that it can't slip down (or up!). Measure the distance from the top of the hose to the existing patio level - it's best to set the hose at an 'easy-to-remember' height, say 300mm above patio level.
Next, pay out the hose until the other end is at the point on the new patio where you want to establish a level. It doesn't matter whether the hose snakes around the garden; it doesn't have to travel in a straight line; you can go round corners and coil it around trees if you wish, as long as the free end of the hose reaches to required level point with no kinks in it, and there are no places where the hose is higher than the supported end back at the old patio.
It's best to drive in another marker stake at this point, and then lash the free end of the hose to this, but only temporarily. You'll be adjusting the level of this end of the hose in a moment or two. Try to make this end of the hose slightly higher than the other end, using a "best guess" strategy. It doesn't matter if it's 50mm higher or 500mm higher, just as long as you reckon it's higher.
The next job is to fill the hose with water. You can do this by temporarily attaching another hose to one end, or by pouring water in from a jug or a bucket, but fill the hose so that the water in the hose comes right up to the top of the hose at the old patio end.
Now comes the clever bit. You go over to the 'new' end, and then carefully adjust the height of the open end of the hose, moving it down a few millimetres at a time, until water is just about to dribble over the top. Because water always finds its own level, the level of the water at this end of the hose
must be the same as the level of the water at the other end.
Mark the level of the top of the hose on the stake, and then measure down an identical dimension as was used over at the old patio end of things, and there you have it - levels established and accurate to around 5-10mm, which is nowt in garden patio terms!
Good, innit! :)