Temperature has a massive impact on seed germination and both grasses and clovers requite a similar temperature to start growing from seed.
Soil temperatures needs to be in excess of 4°C, usually late February in southern UK, two weeks days later in the north. The optimum soil temperature for seeds to start chitting is 7-10°C which is usually reached by mid - late March across the UK.
If planning on drilling leys this spring, wait till the soil has started to warm before sowing seed. Please phone 01608 652552 for advice on your choice of grass and location.