Vauxhall Meriva engine warning light on

I meant to put 2009 as the year of manufacture.

Good name!
'cause you are.
p0136 is for the second lambda sensor,
or 02 sensor.
Which might/may be faulty,
only way to know is read the live data and check the 02 output for voltage fluctuation and frequency.
Or it may be the catalytic converter has failed
Or you can go ahead and swap out what is possibly a perfectly good part!
Oh and the pedal test stopped being reliable in the 90's.
I would code read for faults,
and "IF" P0136 came up I would then test for the right outputs at the sensor and perhaps a gas enrichment test to see if I could get any o2 output change.
Test for a temperature differential between the pre and post Cat temps.
P0136 seldom just means renew the post cat sensor.