Vitamin D status, which is amenable to intervention by safely increasing sun exposure or vitamin D supplementation, was associated inversely with BP

The relationship between vitamin d and blood pressure is probably driven by improved calcium uptake. In order to maximise calcium absorption from the diet vitamin d status has to at least 80nmol/l.

Most people in the UK are vitamin d insufficient. The average UK winter status is around 40nmol/l (people in their 70's drop down to 23nmol/l in the winter) Each 400iu vitamin D3 tablet will raise your status by 9nmol/L. It follows that to achieve a status over 80nmol/l (this also reduces colon cancer incidence by 72%) requires around 2000iu/daily vitamin D3. Risk assessment for vitamin D details the safety of up to 10,000iu/daily/D3

This is why blood pressure naturally is lower in the summer (more sunshine more vitamin d3), when you spend time nearer the equator or when you spend time at higher altitudes (nearer the sun/less atmospheric pollution for the UVB rays to penetrate.