@Shooting
There is one on the cat, but that is the "Downstream Oxygen Sensor" and I've not seen any DTCs from that one.
Some of the techs I've spoken to about this think it may be something elsewhere in the engine that is causing the mixture to be wrong and that is being picked up by the sensor, but its incredibly hard to fix when you can't make the engine fault, as it feels entirely fine to drive. I've had several mechanics look at it too, and non of them can find anything wrong! That's why I thought I'd just get rid of the P0134 DTC so that (a) It doesn't bother me anymore and (b) keeps the EML off.
Peugeot say this can happen if you fit a non-original O2 sensor but I'm a bit sceptical about that tbh.
Also... for some reason I can't now clear any faults on the ECU with any OBD tool, including Peugeot's Planet. I've no idea what that is about so I'm thinking about just replacing the ECU as they are plentiful on eBay and cheap (c £30).
Thanks again for all your help mate.