I just went hunting to the FSA website and saw it's not on there.
So it's a withdrawal not a recall.
The legislation in the UK is around "knowing or have reason to believe" something is not food safe.
As I understand it from what you've said above, what has happened is the supplier with the unfortunate acronym have contacted you to say there "may" be an issue in some product they have supplied but have not recalled it. They are wanting to withdraw it for more data etc.
Now, this is where I'd kick off my crisis team. The supplier has approached you with a softly softly intent not trying to set hares running... But the problem is you've used some of the sugar, is that correct?
This is where I'd be alerting your crisis team (and the responsible person needs to alert your insurers as well) but the first priority is, in my mind, if your crisis team agree, to go back to the supplier, explain some of the sugar has been used AND GET THEM to explain how much of a risk they think it is. In the meantime, get everything ready, everything traced and I'd be putting nothing else on the market made with those batches until I knew more but I wouldn't recall yet until I got a clear answer from the suppliers.
This is a consumer risk issue and an insurance issue but the latter is on two fronts.
For the American readers, no this would not be classed as "adulterated" stock and it sounds very much like the supplier has not contacted the competent authorities. It might simply be the supplier doesn't know or it might be a slight fishing expedition to find out how far product has been used.
As for if it's a risk when it's been through your metal detection? Good idea to get a sample of the material. But if you find it DOES detect in your equipment and you've not had any activations, all that will do is make you worried your staff haven't followed the procedure. Sorry I'm just warning you... You might then still decide to pull it back depending on how widespread and how much metal could be in it. In the background your finance guys might want to be checking how much liability the supplier signed up to in their contract.