Filling the gaps (Hawthorns)

Depends how far apart the standing ones are, how low the branches are for laying and if you require this to be stock proof, just denote a boundary or somewhere in between.
If they're fairly close together and you have some decent uprights that are fairly straight you might get away with laying it. Remember that the sap needs to flow upwards so always make sure the cut is as low down as possible to allow you to keep the layed bit of hedge as close to the ground as possible but still running uphill.