Ceramic kitchen 1.5 bowl sink - hairline glaze cracks around plugholes - cause?

[Spreadsheetman]

My usual (very good) plumber fitted a Rangemaster 1.5 bowl ceramic kitchen sink for us back in April and it has been successively developing more and more hairline cracks radiating out from the plug holes. Mostly in the drainer sink, but still in both. We used all the supplied (plastic+rubber washers) fittings and used clear silicone to seal to the ceramic as usual.

 

I have been wrangling with the manufacturer via the supplier and they are claiming that we must have overtightened the fittings which has cracked the glaze and then thermal cycling is causing further cracking. So of cause, they are accepting no responsibility.

 

Anyone got any suggestions? We used the supplied fittings, my plumber has done hundreds of sinks and I was helping and it all looked correct to me. How easily are sinks damaged just by attaching normal waste fittings? I wouldn't have thought it was possible, especially since the fixing threads are into plastic anyway.