At long last I've decided. I've ordered a shiny new DNUK Deskstar, desktop PC to replace my current DNUK Workstar system which has reached the end of it's useful life as a front line system. The new box is at least four times better in every respect: it has four cores compared with one; 8 GiB of RAM instead of 2 GiB; ten times the hard-disk capacity (which is also faster) and a solid state drive; hardware virtualisation and a drastically superior graphics card. It's also a Intel based system, all my previous DNUK boxes have had AMD processors. It will cost more money than the system it replaces, but a system of similar price (accepting inflation) would not have been sufficiently faster or balanced to make it worth buying.
I've also started the process of migrating this server off the current Bytemark virtual server onto their new BigV platform. The new system is faster, more scalable and slightly cheaper. It also allows me a pain-free upgrade to the latest version of Debian.