Thanks for the replies.
A fire extinguisher isn't a consideration, there's nothing in place to make it feasible. It's an industrial burner, I'm just trying to improve the existing control system (the main priority is the inclusion of a timer for auto shut-off).
I'll look into the diodes, but I was planning on using "off the shelf" arduino compatible relays (such as the ones found on ebay based around an Omron) - I figured it would be easier and plug and play?
I'll get a handful of resistors etc. too - and fuses is a good idea, thanks.
One other thing I'm considering - would I be better to use a RTC for timing purposes, or would it be overkill? The only timing would be for the oven's cure cycle (15 mins), and possibly a cumulative timer for burner on high/low.
Thanks again, I always find someone else will see something that I haven't.