UPDATE - DS is fine.
We rang 111, and were awaiting a call back. DH also rang 2 of our friends, a GP and a hospital pharmacist. Both said not to worry liquitabs are immediate a&e, normal liquid is a&e if in eyes or if symptomatic (ulcers, redness, burns, rash, vomitting or diarrhoea or difficulty breathing). Whilst DS was on the phone I posted on mum's net.
The bottle was in the recycling, empty and having had sluice out, but with some sticky bits round the rim, the amount ingested would have been tiny. If it had some in, I'd have taken him to hospital immediately.
111 called us back within 35 minutes. A pharmacist went through the exact product and it's ingredients and confirmed that unless symptomatic he didn't need to go to hospital.
He slept in bed with us last night so we could keep an eye on him.
All cleaning products are locked away normally, and from now on, any ones for recycling will be going straight in the outside bin!
I went in to work this morning and reported the incident to my manager (I'm a social worker!).