LONDON (Reuters) - Oil and gas producer Cairn Energy on Friday reduced investment plans by about a fifth, following the fall of oil prices to less than $30 a barrel.Cairn and its partners, including Woodside and FAR , were assessing "substantial initiatives to reduce and re-phase" investment in the $4.2 billion (3.4 billion pounds) Sangomar oil development project in Senegal, Cairn said in a statement.Sangomar is expected to produce around 100,000 barrels per day with first oil scheduled for..