Thanks for all the helpful replies.
I think perhaps the waste was 1cm smaller than the hole, and rather than let me know I need to get a new waste, the plumber filled the gap with sealant.
He certainly did some other strange things, like running a radiator pipe along the skirting board...
My plug got stuck due to sand, and it wouldn't budge, so I removed the waste in order to replace it.
I found the waste hole in the ceramic sink had about 1cm of silicone all the way to below the sink, i.e. around the threaded pipe all the way down. I only discovered this after I'd hacked about...