Thats a good point, keypeg. For me I suspect that the experience of playing guitar and having to tune it all the time was significant in my ears development..
..I've also done plenty of singing, despite thinking I'm pretty terrible at it - but that opinion of myself is a result of me having a discerning ear.
I could probably devise a way to learn relative pitch (practice ideas etc., though to be fair, the way I did it was just to rely on my ear to figure out how to play things.. rather than try to develop ear skills in isolation from playing.
..pick an easy piece, not too many notes, simple harmony.. so anything grade 1 or lower. Listen to it over and over and start trying to figure it out by ear - which will initially be by trial and error for the first note.. but if you can figure out the first note, then identify whether the next note is higher or lower you're already using relative pitch.. all you need do is refine it.