Help unclogging an old cement sink plumbing advice

Help unclogging an old cement sink Hi all,

I have an old school cement laundry sink which is clogged. I unscrewed the pipe underneath to try to clear it, and I was able to get some crud out of there. But it seems the majority of the blockage is higher up, in a section concealed within the cement where the two drains get combined into one. I've tried to see if the strainers can be removed with needle nose pliers, but they seem stuck fast. We've tried drain cleaning chemicals, but it only resolves the situation temporarily before the clog returns. Does anyone have any idea how I would go about unclogging this sink properly?

Many thanks,
Simon

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