Thanks for the link - that's quite informative, in fact the most informative I've found on the Sigma flash. It seems they are both similar in capability, but it takes the advice to a novice from some helpful to show the way. Much appreciated.
You might have just swung me there as the budget is around £160 - hunting around it seems the Sigma and the Speedlite 430 retail for similar prices. I just need to convince myself that an EOS400D with a Sigma lens and flash is still a Canon!