faulty petrol reading

[iain audi a3 1.8T]

hi all, got into my car the other day and my petrol gauge is saying iv got a full tank and trip computer saying iv got 320miles remaining, i havent as iv thrashed it for a full day now and it hasnt moved, any ideas on what this fault would be please? thanks

 

[fingermouse]

sensor in tank under rear seat, bit tricky to get to but it will probally need a clean.