However it happens after the managed code had been unloaded. The error
message I get says

The exception ( 0xc0020001) occurred in the application at location 0x7c81eb33

this appears to be happening as it's trying to unload the activeX control,
which is then trying to call back into managed code that is no longer there
...

I'm hosting the control inside a System.Windows.Forms.UserControl, if that
makes any difference, and the Dispose is getting called.

This is driving me crazy. The same Html renderer worked under in our
previous app used in almost exactly the same way, but now no longer does.

thanks