This is a piece of string question in some respects. The important consideration with runner beans is that they prefer warm soil and will be quickly killed by any level of frost. In the southern parts of the UK it can usually be sufficiently warm in the latter part of May. This is obviously not a hard and fast rule. A reasonable test is to dig a small hole and place the back of your hand in it and against the soil. If it feels a little warm, as against very cold, you will be safe to plant your bean seeds. The important thing with most seeds is that you do not want them dormant in the soil too long after sowing, hence sufficient warm and sufficient water to encourage germination and growth and to substain the same.