Thanks kamran. You swung it for me.
I reserved a set and went off to Comet to collect but when I got ther I found they had th x-230s for £25 too. After a lot of indecision I tried them both out and have to say the 230s sounded better balanced. There seemed to be more of a gap between the low and high frequencies with the Z4 but I'm no hifi expert. However, the minimum bass level on the 230s was still too loud (I want to at least be able to be sociable when I choose to!) whereas the Z4s bass could be turned right down to zero. Plus the remote on the Z4s is really nice and has bass control on it so I went for the Z4s...only to find they had no stock! Decided to pay for them anyway at this price and will pick up when they next come in.
So it was a bit backward but I went for my own deal in the end!
Edit: Posted the X-230s as a deal too as it's much cheaper than even the best internet price, and they're supposed to be very good speakers too (
http://www.hotukdeals.com/item/37081...-2-1-pc-speak/)