OK, so I now have a 500GB SSD drive, capable of ~5Gbps but limited to 3Gbps by the SATA interface.
Has it made a difference? Too right it has! The PC has life that it never had before. To be fair I have also started with a fresh installation of Ubuntu (it was previously Kubuntu) but I have a new laptop running the same Ubuntu release which by comparison runs like a dog despite being much newer hardware.
My feeling is that multi-tasking is where it really lifts the performance: as soon as the old disk had one process needing data from one side of the disk and another needing data from the opposite side it was grinding to a halt, but the SSD is truly random access.
It's too early to justify upgrading the laptop as well but so far it's been way better than any other performance upgrade I've tried.