For tasks - is a company reference key required? I have a custom record which does not reference any company but when I try and create a workflow that creates a task for this custom record I cannot attach the task because am getting an error asking for a company reference key.

I don't think it should be required - checked one of my accounts and it doesn't appear to be

it's odd becasue i can create a task manually without a company but via workflow I can create but not attach to the transaction.

that is very odd

My guess is you are inadvertly setting that field to a bad value.