If, on www.crucial.com/uk, you go through the process to select memory for your iBook G4, and then select the 1GB stick, you will find it is £85 including shipping and VAT, as opposed to a normal price of £125. I just ordered some, and it seems a good deal for iBook users wanting 1GB more RAM.
Just an addition for those who can't stretch to (or don't need) a GB of RAM even at £85 I just ordered a 512MB Kingston stick for my mother's iBook (complete with lifetime guarantee) from ebuyer.co.uk for £46 (inc delivery). Most impressively, I ordered it a 11pm on Wednesday night and chose the cheapest saver delivery - expected delivery Thursday 16 June. I got home tonight to find that they'd delivered it... I'm impressed!
For me, eBuyer delivery is the among the worst on the web... perhaps they've improved since I last used them 4 months ago! That's really good news. Not bad pricing either! Crucial has always been good for me.