There was mention somewhere (possibly another thread, possibly a thread on another website entirely) that mentioned avoiding French diesels because of having to faff about with AdBlue. As I've mentioned I'm out of the loop with modern(ish) cars, so what exactly would this entail? More importantly is it a bit of a red herring?. I'm quite keen on the Peugeot 207 SW and the Renault Clio Touring, but most of them seem to be diesels (especially the Pugs) so is this something better avoided?
Very generally from 2016 emissions rules were tightened up, euro6 was introduced (the exact date that changes took place varies a few months by models prior to deadline). The earlier diesels usually do not meet low emission zone rules (LEZ rules vary by city)
When euro6 was introduced most couldn't meet it and adding ad blu (a synthetic urea, or pee if you prefer) needs to be added to separate reservoir was part of the solution. Early ones often had these in odd places although later smaller blue filler caps were added near diesel fuel tank. Even with these most still couldn't meet euro6 and all sorts of temporary weaker standards applied for few years (will see something like euro6d temp). Over time complicated and hugely expensive to repair extra emissions gear was added to diesel engines.
Diesels were popular for few years, there was a lower tax for a while as it used CO2 emissions ignoring total emissions (diesels are higher NOx and particulates). But now everyone realises all the extra emissions equipment is very costly (often about £2k extra new for diesel over petrol version), and the new equipment is prone to clogging up if don't do a minimum 30-45 minute journey regularly to get it hot enough to do a regeneration. Less than 1 in 25 new cars in UK is now diesel (excluding diesel hybrids which are obviously even more complicated).
The reason why you are finding more diesels on sale is they are harder to sell and stick around unsold, the petrols sell quickly, people now opt for easier to maintain petrols because worried about £1500+ bills to sort clogged diesel emission filters.