How to unblock Belfast sink overflow

[bakingdemon]

The overflow to our sink is blocked. Has been for a few days. We have tried Mr Muscle drain stuff, slinging kettles of boiling water down it and DH tried caustic soda this morning. Nada. Any ideas?

www.wickes.co.uk/Wickes-Flexible-Coil-Drain-Cleaner---4-65m/p/500594?sv_campaign_id=103504&sv_tax1=affiliate&sv_tax3=Future+Publishing.&sv_tax4=0&sv_affiliate_id=103504&awc=1563_1617625049_6888a429eef0d7a307c4273e952200de&utm_source=affiliate_window&utm_medium=affiliate&utm_term=103504&utm_content=deeplink&utm_campaign=TechRadar

These are good, I cleaned my shower drain with one