Thanks for the suggestions. (10,000GBP = 15,000USD)
Netapp looks good, I was looking at the FAS2xxx series, be interested to know if anyone has any experience with these. We had an Netapp before and it was bomb proof.
The Isilon's have generally been fine (apart from a horrible bug where they didn't work with CentOS guests on ESXi 4.x). We just need to re purpose them to add them to another cluster.