By that response i gauge that its not easy or cheap so probably not a suitable business idea?
Well, a 172 comes in at about £1.2mil/carriage, a 185 today would be £1.5mil, a 'hybrid' (oh god how I hate the term) would come in at something around £1.6mil from Bombardier or £1.9mil from Siemens.
I'm not doing anything more complex here than extrapolating the cost of a Voyager Carriage £1.1mil and adding on development, short run, etc costs.
The problem comes when you consider the minimum length for any such unit would be 3 carriages, two with generators and motored bogies and one with panto, transformer, etc. Two 750hp generators should be plenty for a 3 car unit. But then where is the market for new diesel, let alone a whole new expensive development that may not be needed in 10 years anyway? Especially when with Derby's development costs, or Siemens' short run costs you'd be looking at such a unit costing nearly twice as much. In 30 years time this will have a market, when the diesel supplies are much more scarse and TOCs want to eek out every last Watt from the grid.
There are better ways IMO to address the stock shortage problems, and personally I'd say that a big one of them is the production of a new series of LHCS that can be diesel hauled, electric hauled or both.
Reigonal branch services can be hauled by a freight locomotive, god knows we have plenty going spare, mainline services over 75mph (diesel) can be hauled by a new series of Eurolights running at 110mph, easilly do-able and will be needed for years to come on the GW Cornish services etc. Anywhere that frequently uses formations of 5, 6car or longer DMUs will move over to LHCS, the HST replacements would be these as well, possibly seeing if one can push the speed on Diesel up to 125 with two locomotives. (Or simply change from Leccy to Diesel Loco)
If this was combined with continued electrification, one could simply move all of the long distance scottish services over to LHCS and release 156/8 and 170 units to replace the remaining pacers from the current electrification plans, and increase capacity.