Hi Giles, since I'm in the thread and also bought a lambda sensor this week for my 1.6 Sigma....
To narrow the search for you I have selected a Ford model that has 2 liters with 146 hp and was installed in the Ford Mondeo from 2000 to 2007. I would be very surprised if it was a different engine than the Plus 4 Duratec. I have chosen Bosch as a reliable brand, but Denso or Hella are also ok. I wouldn't save £10 and buy the very cheapest sensor. May be you copy the parts number of the sensor and try it in an UK online shop. At least I recognize the same shape of the plug in the photo in the link below as my Ford engine has. To be very sure just measure the length of the cable at your current failed sensor. Note, I wanted a 40mm cable but the length over all including plug and sensor is ca. 40mm. Anyway it is long enough in my case. It is important to measure the needed length of the cable because the enginge has in the Morgan probably another position to the loom vs. a Ford car.

Control sensor in front of catalytic converter:

https://www.autodoc.de/bosch/1149000


In the fold-out list at the bottom of the link, you can see that a myriad of different Ford engines and models are equipped with this lambda sensor. This means that you are technically very probably buying the right part, and save some Money in comparison to Fomoco OEM prices which use also Bosch, I think.
I even once bought a so called Bosch "Universal" lambda sensor for another car. The difference is that this universal sensor comes without a connector with loose cable ends. This means on the one hand the proof that the sensor itself does not need any special compatibility requirements in most cases (unless you need a wideband lambda sensor which is not the case with your Plus 4).

But I advise from my own experience against the choice of a universal probe! The reason is that the fine silver wires need absolute 0 Ohm connections. My soldering skills were not too good and the sensor did not run 100% correctly. Either someone has a set to cold connect...but much better to have the right connector mounted when buying the part.

'14 4/4 graphite grey