Well seeing as this thread is here... my mpg has fallen from over 80mpg - to 60mpg and this tank fallen to just 22mpg
mk1 1.1L 8V 50k miles well serviced & looked after
My thoughts are a cracked cylinder head, could this result in the poor mpg?

?leaking fuel filter see othe rposts re this...

Guesswork or do all the usual diagnostic checks and report the issues found in a new thread.