The qualifying entry ids seem to work in a way that means the discount is only applied to a matching product if more than 1 of that particular product is purchased rather than more than 1 from ANY of the qualifying ids.

i.e.
If my discount is a buy 2 get 50p off and qualifying ids are 1 and 2 then the discount is applied like so:

Buy 1 and 2 = discount applied
Buy 1 and 1 = no discount.

Is this how it’s *meant* to behave and if so, would it be possible to modify a plugin to apply the discount if any products from the range of qualifying ids matched?

The qualifying entry ids seem to work in a way that means the discount is only applied to a matching product if more than 1 of that particular product is purchased rather than more than 1 from ANY of the qualifying ids.

You’re right. This is how it was meant to work.

would it be possible to modify a plugin to apply the discount if any products from the range of qualifying ids matched?

Yes. It’s something we’re working on for another customer right now as a matter of fact.

Is this a major change? I’ve had a quick look at making my own discount plugin by modifying the Buy X get X plugin but so far I haven’t got very far.
Is this qualifying ID behaviour part of the discount type plugin or the parent discount class?

I would consider it a pretty significant change since it requires changing the logic around.

The qualifying ID behavior is part of the discount type plugin.