(ShareCast News) - UK Oil and Gas (UKOG) said it has begun working on a field development plan after its extension plans for the Markwells Wood-1 well in West Sussex was given the green light by local authorities.The South Downs National Park Authority has extended permission the well until 30 September 2016, the London-listed company said in a statement on Friday, adding it had started working on a Markwells Wood field development plan ahead of submission to the UK Oil and Gas Authority (OGA).UKOG, which operates the Horse Hill site near London's Gatwick Airport, said it requires the authority's approval to move the PEDL126 licence for the site from its current exploration phase into an 18-year production period."This is a key step to enable the company to move forward with its plan to develop this UK onshore oil field to add further oil production to its portfolio," said group chairman Stephen Sanderson.UKOG shares were down 2.71% to 1.43p at 0841 BST on Friday.