Well finally borrowed an USB disk big enough - though the actual backup of the playstation ended up at 8GB it complained about fitting on my USB drive. So aside from a simple backup of the old drive via the XMB tool. Replace the old 40GB 2.5” SATA drive with a nice shiny 320GB model (and then restored from the USB drive)
Quite a few guides on the interweb about how to do that, but I’m still slightly shocked that Sony are using standard parts these days!
Time taken: 2 hours (mostly waiting for backup/restore)