Oil is bought in US dollars. £1 = 1.17$ on 12 sept, !.13$ on 22 sept, now its 1.09 $ . Or put another way ,at todays $ price for a barrel of crude , it would have cost about £73 in August and £79 today.
(Maybe the quasi chancellor will one day say " here is the sick squid I owe you" -pay me back over the next 10 years. )
Ho hum.