In response to my own question, it seems that there are 3 real options. Digital Workshops, coincidentally a distributor of PSP offer a budget solution called E-Pen (serial only). Then there is Wacom who also offer a budget solution called the Graphire (USB) which is slightly more expensive but with the advantage of USB and having a wireless stylus. Wacom also offer a 'professional' solution called the Intuos (USB or serial) which comes in a range of sizes (A4,A5 & A6).
Rob.