Colin Wilson wrote:
any others, as not a lot of these are workable due to the caustic soda

Tell your missus to pay for the a call from a drain company...

It might be worth investing in a set of drain rods with a spiral so
you can hack away at the blockage from downstream.

You really will have to be careful as regards the caustic soda though,
as I believe the blockage can be prone to clearing quite quickly once
breached. Goggles would be advisable as a minimum anyway at a guess !
trouble is the fat appears to be at the top of the ubend, so access from
the man hole end will not touch it(i.e just below the pipe, and from the
drain end its quite a route through caustic soda,