How to seal leaky belfast sink grating?

[Bonobius]

I have a 2 year old Belfast style farmhouse ceramic sink with a basket strainer waste fitting. The water leaks through the side of the grating, draining the sink far too quickly. We've tried sealing with silicone, Boss Blue, and plumbers mait but after a week or so the water works it way through again. There isn't enough space to insert a rubber ring between the grating and the ceramic - and the original plumber just used silicone when installing the sink anyway. Do we have to live with this or is there a better sealing solution?

I'm not a plumber as you can probably tell and would rather call in a professional than DIY but we're not having much luck at the minute!

Thanks.