Codswallop wrote:

Caustic soda solution might have to be left all day or over-night for a
really bad blockage, even though the label says leave for 30 minutes. Have
you tried vigorous sink plunging? It is important to block the overflow
whilst plunging otherwise a lot of the suction and pressure will be lost.
Can you insert the flexible metal drain clearer up the outlet end? If all
else fails and the waste pipe is connected with push-fit or compression
fittings you may have to dismantle the pipe further along and clear the
blockage. But if you can't get at the pipe or if it has welded fittings then
its back to caustic soda and the plunger. You have got to be more determined
that the blockage will move than the blockage is determined that it is going
to stay put.

The pipe is not really accessible. I haven't tried plunging though.
I'll try the suggested Screwfix product.