Not sure if you know this but the challenge xtreme petrol strimmer 30cc is basicaly a Spear & Jackson 30cc strimmer they even use the same model number STG30N i think it is ? they are just just "dressed" differently for a different retailer (Argos is a challenge outlet), but both have indetical working parts.
Also if you go to B&Q the spools off most Ryobi strimmers and Homelite strimmers (under liscence to B&Q) will fit the challenge xtreme 30cc & 26cc, exactly the same spool, i think the part number is H502 ? Dont fret about parts for the xtreme model you can easily use workable parts from other makes. Hope this helps.
kerry.
August 2010