To achieve the best possible results from short term, grass-based leys sown in the spring the leys need to be a mix containing fast growing annuals and short term species like westerwolds ryegrass, crimson and red clover and vetch.  

Sow early in the Spring into a well consolidated seedbed while there is still some moisture in the soil and ensure there is sufficient N, P and K available for the newly emerging seedlings.

Leys like this will work best on moisture retentive soils. For those on dry soils autumn sowing is best as this allows plenty of time for a root system to develop before the dry weather sets in. 

If a long term ley is required then almost any mixture can be drilled from now onwards although yields in the first year will be lower than in subsequent years.