Has any work been done in the tank recently? Jobs like fitting the orange clip or freeing up a sticking level float? The usual cause of petrol loss in the way you describe typcally results from an incorrectly replaced sealing ring on one of the tank ports. As Peter has indicated, replacing this sealing ring is pretty easy providing certain rules are followed.
You can get the details from this HowTo:
https://www.the75andztclub.co.uk/for...ead.php?t=9108
This covers filter replacement, but the port seal fitment is described in detail.
TC