So I spent the afternoon playing around with gradient nails, and this is what I've ended up with. They're far from perfect, but I'm pretty pleased with them for now. These remind me of a Tequila Sunrise!
I used a base coat of white (Models Own Pro French White) and then sponged yellow (Barry M Yellow), orange (Barry M Gelly Satsuma) and then red (Barry M Gelly Blood Orange) over the top, one at a time.
They could be tidier, but they look fab from arm's length!