Hi Chris, I have the same problem. I inherited my ART30 strimmer which worked ok at first but then made a loud short rattling noise and it shed the whole of the line. The spool i have in it is a RY054 and I too wondered about the possibility of a missing spring. Have you managed to fix yours yet?
On a different note..... how is the Black Country nowadays? I used to live in Bearwood but moved some years ago to the Black Mountains.