At first I suspected some of the modifications that I've made over the last week or so (Skype Video & hardware additions) were the cause of my boot woes: but it turns out that it was a problem that has haunted me before and one that I'd never managed to resolve.
Searching my syslog threw up the strange but persistent error message:
jogga-desktop kernel: [ ##.######] serial8250: too much work for irq18
Where the # are (seemingly) random numbers. This line is repeated numerous times throughout the log and, I reasoned, must at least contribute to any delay. Trawling various bug reports and online forums led me to believe that the problem might be caused by an unused modem: so, I simply un-installed modemmanager using the Synaptic Package Manager.