sticking fuel gauge

[LukeZS96]

This morning I filled up the Zed, completely brimmed the tank till the nozzle clicked, paid €55 for it all, gauge would only go up past half way, initially thought it was a fuel leak so when I got to the next garage I went to fill it again and it took less than €1 for the nozzle to click, gauge still stuck at half way and there is no sign of a fuel leak anywhwere around the filler neck/fuel tank etc.So I am fairly confident that it's either a faulty gauge or faulty sender unit, so. Where is the best place to start and how to test each unit? it was on just under quarter of a tank when filled
Thanks lads