I've lived with this issue for years, but seeing as my mini has finally been MOT'd after 2 and a half years in my garage (just in time for winter!?!), I thought I'd actually get round to sorting it.
If I put a full tank in, then if memory serves me correctly, then it might show half a tank.
I put about £20 in it from empty this week and the gauge showed the level just above the red.
I purchased a new fuel sender a few years back, hoping to resolve it, but made no different.