I think coppice is correct on this one - it's some sort of proprietary MC. I roughed up a schematic of both boards and it certainly doesn't look like this chip is one of the 8-pin toaster controllers that fzabkar pointed me too. Circuitry doesn't look remotely like illustrative circuit in datasheet. Looks like it's running on a 5V DC supply, but I really have little idea of pin functions. I did get the toaster to come back to life briefly after soldering the main board back into place, but then part of one main element stopped turning on despite having continuity and a sensible low resistance. That again looks like a problem with switching the right relays, which is controlled by this daughter board MC. In the end I decided to can it - it will take more time than it's worth to diagnose and more expense to fix than it's worth.
I think this shift to microcontroller based switching is greatly impacting the working life of appliances. I've had failures on a whole range of items and most have come down to MC-related problems. Seems to me the lifespan of these MC appliances is perhaps 3-4 years then you have to throw them out. I can think of only one (a mate's coffee machine) where the fixable fault was a bad diode and relay on the main control board. In contrast, I have a completely analogue-switched expresso machine that's been going strong for near 20 years and I've fixed several times without drama. Hopefully I can buy a non-digital toaster to replace this expensive dud.