This is the replacement to the Asus Eee 701. It's improvements include:
8.9" screen instead of 7"
1024 x 800 resolution instead of 800 x 600 (much better for surfing)
1GB RAM instead of 512MB
20GB SSD instead of 4GB (You can also order a version with XP, but it only has 12GB)
I suppose the main question is whether this is worth the extra £100. I would say it is. an extra 16GB of storage is worth £40, and you don't sacrifice the USB or SD card slots to get it. The extra RAM is another £20 of value, but the main benefit is the larger screen. There were also rumours that this would have touch-screen functionality (though the product spec doesn't mention it).
I would guess these will sell out fast, so get your orders in! :)
EDIT: Available for the same price from Play, but 5% voucher and quidco make it cheaper.