One of the reasons Chiltern are looking at Loco's is that where an DMU has more than 5 engines, it starts to become more economic to run the service as loco & coaching stock. There's a piece in the July edition of Modern Railways that mentions this.
This tends to only hold for higher speed DMUs (those approaching 125mph) and not this rather slow regime (its almost the Express Sprinter/Turbostar regime).
As an example a five coach Turbostar derived DMU (whether it has end doors or not) with similar usable internal space, weighs in at ~210t compared to the 133t for a Cl67 and DVT alone before we even consider the coaches.
The total weight comes out somewhere in the region of 330t with the same five coaches of internal space (using the IE
De Dietrich as an example of modern rolling stock at ~39t each).
The Turbostar would have Cl172 derived mechanical equipment with 485hp per carriage, which gives a five carriage formation 2425hp or 11.54hp/t.
Meanwhile the loco hauled formation has ~3200hp or 9.70hp/t.
If you were only interested in loco hauled performance you could have a formation consisting of four motor vehicles and one trailer, which reduces the maintenance considerably.
If they can obtain a loco that uses less diesel than the 67's & costs less to maintain, I'm sure they'll be interested. My concern is that notwithstanding the excellent refurbishment of the Mk3's, they are getting on in years & won't last forever. Will we ever see new coaches built to work with the loco's?
A Eurolight is likely to improve the weight issue to a degree but it does so by adopting multiple-unit type engines rather than a single power-plant.
It is far more likely that these would be sent to work on the sort of freights that Cl67s work currently after the Mark 3s are scrapped.
There will be no major return to loco hauled formations in the UK, they just can't compete.