There are several routes to both increasing species-richness in a poor sward (restoration), and creating a meadow from a previously arable field (creation). Changes to the way the meadow is managed may be enough to encourage more plants back into the sward, especially if there is a good seed source nearby, and if there have been damaging changes to the meadow in the past that can be reversed. Some examples include altering water management or altering the cutting and grazing regime to encourage a species-rich plant community.
Spreading green hay (hay cut from a more diverse site just before it is ready for hay, and collected and spread without drying or wilting) onto a species-poor area is an effective method especially if you have a species-rich meadow nearby and the soil structure and water regime on the receptor site has not been significantly altered.
Alternatively spreading seed collected from a suitable donor site or purchased from a supplier has also proved successful.
Further information on practical methods of meadow restoration can be found by following the various lnks below:
Technical Information Notes - Natural England