I've been trying to find this one for a few days without any luck. Just a few details to go on with this one - the characters are mainly Buffy, Spike, Giles and Willow. Buffy and Spike are in search of something that is changing adults into children (I seem to remember a scene in a pub, which makes me think the fic is based in England). Willow and Giles are chasing after Buffy and Spike with a spell on their car to prevent them from being overcome by the whatever it is that's turning people young. In the end I think it's Giles who is the Big Bad, trying to split Buffy and Spike up again.
Any help will be gratefully received.
ETA: Found thanks to
slaymesoftly - it's 'Forever and a Day' by
lilachigh (link in comments).