Toyota brought out a fully fledged Prius over 23 years ago.

In the last 2 years or so Dozens of brands have bought bolt on low tech so they can have a a tick in the box for the specification and a Hybrid badge on the boot / hatch.

Agree with that.

We test drove a Corolla 2 litre and found it brilliant in town where it was happy to drive silently on electric providing you were gentle with the throttle. On a 70 mph dual carriageway I was surprised when the ICE dropped out and it drove on electric power alone on a flat piece of road for quite a distance. It was spoilt somewhat in the Peak District when at times there were plenty of revs and not much action but that's not unusual for most auto's.

But these mild hybrids would do none of that. The mild hybrid bit is there only to enhance the stop/start function, they will not drive on electric power. Even the loonies in Government accept this which is why Mild Hybrids will be banned from sale at the same time as ICE cars. No 5 years stay of execution like hybrids and plug in hybrids. I think the deciding factor is how far they will drive solely on electric power which for a mild hybrid is ZERO miles.

Does it reduce emissions thereby giving some tax advantage to manufacturers?

I think it reduces the CO's for the manufacturers thus reduces the fine they get for exceeding the limit set.

Like old style stop/start its a con.

Why there is a tendency to bundle a hybrid even if just a token hybrid and offers no real benefit as such?

Does it reduce emissions thereby giving some tax advantage to manufacturers?

Yes, and that is the main reason for them becoming much more common, to the point that it will probably soon become difficult to buy a car which is not, at the very least, a mild hybrid (MHEV).

MHEV's are a relatively cheap means for a manufacturer to bring down their overall fleet emissions, that is to say, taking all the cars they sell into account. Failing to meet the very stringent targets will result in truly huge fines, so it is very much in the manufacturers interest to do so.

Of course they are just a stopgap, because come 2030, no new MHEV's will be able to be sold (just as with ICE) because a MHEV can't run on electric alone.

With regards to their effectiveness, as with 'full hybrids', a certain driving style is best. Driving in the aggressive manner typical of the modern driver, any benefit would probably be down to quicker acting stop start along with the ability, on higher end MHEV's, to 'coast'. But for other, 'better' drivers, the benefits could be much more worthwhile. For example, when What Car did their 'real world mpg' test on the Suzuki Ignis MHEV, it returned nearly 20% higher mpg than the non MHEV version.