As the party is on the 10th then do try Hawkin's Bazaar - the site says anything ordered at the moment will be despatched w/c Jan 5th, they do express delivery (ie next day) for an extra £6 and by then services should be back to normal.
Failing that, Woolies have lots of multi-packs of party bags. Matalan is good for girly things - multi-packs of stretchies etc. Also try Claire's Accessories. Don't know what your budget is per head - can imagine for 40 (are you mad, woman?) it's not very much! - but you could fill up with fun-size chocs (Lidl's own-brand are good).