Hi - I engaged a local plumber to fit a new tap and slotted waste to an existing sink. It’s 15 year old and I assume it didn’t leak when first installed but can’t be 100% sure of this as has leaked last few years. He used ptfe tape and plumbers mate. Fitted my new waste twice and then fitted a different new waste that he had spare in van but couldn’t get a seal. Tells me it must be the sink (no cracks) and I will need a new sink. Bad plumber or is he right? Advice welcome please. Thanks Iain