A sample size of one is an anecdote. The plural of anecdote is anecdotes, not data.
Culzean can offer said experience as his anecdote / opinion / belief, as can Hobo and mkjazz . So far we have 3 consumer testimonials, of which:
One says that it "turned out tesco hadn't cleaned out its fuel tanks and their ordinary unleaded isn't the best anyway", BUT follows up with "i've only used bp/ shell or
super from tesco. no further probs on that score."
One says supermarkets "don't bother putting the cleaning and other additives into it"
Another refutes the supermarkets claim as "completely untrue"
Read into that what you like.
I too remember the "Formula Shell" fiasco. Also remember Nikasil plated liners having problems with high sulphur fuel damaging the plating. No retailer whether supermarket or oil major is immune to fuel problems, e.g.
http://news.bbc.co.uk/2/hi/uk_news/northern_ireland/8475859.stmFor my unsupported anecdote tuppence worth, it is my opinion that dodgy deliveries occur from time to time and you can get a duff tank of fuel out of it no matter where you go.
I don't think it's the OP's particular problem though.