Mk1 Forscan HO2 sensors and EML

Hi all, spent quite a while browsing the forum in recent weeks so figured I would join.  Just picked up a 52 reg Focus Zetec Hatch in great shape and well looked after  150k miles but not been driven much in the past year, less than 1000 miles.  Passed MOT a few weeks ago, no advisories except a coolant leak which I can't find easily so have booked into my local indy garage to have a look.  I simply don't have the space, skiils, or tools to be more than an armchair mechanic unfortunately...

Anyway the other night as I was driving (first time up to 60mph since I bought it, before that was just a few hours of city driving) the EML came on.  Had a look via the ignition/trip odo method and got code P0420.  Then hooked up Forscan for an HO2 reading and came up with what I've posted below.  What I don't get is if the cat needs replacing (downstream graph seems to point to that?) then why did it pass the emissions test less than a month ago before I bought it?  What other reasons aside from faulty HO2 sensors could cause that code to appear, and I assume if sensors were faulty, a different code would appear instead?

Graph shown at idle (yellow box), at 2500rpm (blue box), then foot off throttle and back down to idle again.  Engine was up to normal temp and had been running for about 10 min beforehand, parked in front of the house.  Have not done the diagnostic whilst driving around (yet).

Just wondering what to the ask the garage to look at or look for when I go.  I've cleared the DTC codes for now (there were some that had obvious causes unrelated to this Cat efficiency one) so will see if EML comes back on tonight before taking it over to the garage tomorrow.  

Thanks in advance.