uk oil & gas investments described discovery as ` world class ' last week .
they claimed the site in sussex could yield up to 100 billion barrels of oil .
company 's share price increased by 200 per cent following ` breakthrough '
but it has today admitted there may not be as much oil there as suggested .
they based their estimates on the 55-square-miles they have licence for .
it makes up for less than two per cent of the entire weald basin .