Hi op
Are you able to get a pink Dye test?
I didnt like to comment at first and rain on your parade. Then i see your update.
Its a sainsburys test right? If so could very possibly be unrealiable as i had very similar and was defiently not pregnant ( bleeding heavily but undergoing treatment so needed to test to confirm)
They are well known for evap lines unfortunatly if you look up the reviews.
One way is to take the test out of its casing as you see it then if there is a line as it. Best thing is though is to test with another test thats pink dye. Good luck in whatever the outcome is
Xx