Ford club

That's true, trouble is I currently don't subscribe to data roaming on my PAYG iPhone, preferring to use local Wi-Fi networks instead. I imagine I may reconsider my options. My wife's car has data roaming on its Wi-Fi hotspot with Ubigi, £58 per year for 3Gb a month. That seems to be plenty. Is the Fiesta locked into using Vodafone, or is it possible to change the SIM and use another ISP?

No problem.  2007-2014 Mondeo is known as Mk4 in the UK. The 1.8 TDCI in those all came without a DPF as standard for the UK market.  (However, there was a DPF retrofit available, so there is a very small chance of one having been fitted, depending on who the first owner was.) As for water leaks, I can't recall any common areas for those on the Mk4.  Though of course, I won't suggest that none of them ever leak. The weak points for that engine are the fuel injectors, the DMF (flywheel) and they do use both a dry cambelt and wet cambelt.  Often it is only the dry cambelt that gets replaced.

If it's exactly the same part number as the old one then suspect that it's the immobiliser system still trying to talk to the old car. You will probably need to reprogram the keys to the new PCM etc or even get the cluster and keys from the donor car too

Yes, it does matter which type you use.  They are not all the same.   I hadn't heard of Maddox myself.  (Used the PAT fluid from eBay with the green label).  But the website below shows how to check which one you need.  Peugeot/Citroen continued using Eolys when Ford changed to coated DPFs around 2009. Maddox 1044 (EOLYS 176, Infineum F7995) DPF additive (2nd generation), particle filter additive, 3lit. purchase, prices: 21 364 Ft. Ft. (kauffer.hu) (It's in Hungarian - but my browser translated it successfully)

Try updating the software in the cluster, have a look what is available. That may be a glitch