There has recently been an increase in Saudi diesel production, which has dropped the diesel price on forecourts. That price had been driven up because most diesel used to be sourced from Russia. Historically, domestic refineries struggled to keep up with diesel demand in the UK, which meant a lot of diesel was imported at higher cost - this is why diesel has, for a number of years, been more expensive in the UK than petrol.
Just remember that 65% of the cost of the fuel you buy is duty that goes to the Govt coffers... Currently that is 52.95p per litre of petrol or diesel - which does not change however prices may fluctuate - plus 20% VAT, which does vary.