Well first all if there is to be a new build of DMU's while it might be more difficult to get the things financed I suspect a straight DMU will be signifcantly cheaper than a Bi-mode.
I suppose a straight DMU would be cheaper, but you have to weigh that against the chance of electrification making them redundant. If the engines in a bi-mode are leased for 15yrs and then they can be converted to EMUs it may or may not be a safer bet than a straight DMU. In the case of some of the IEP routes I would hope for electrification in less than 10yrs, but a slightly less high-end train could well find non-electrified homes for a good while yet.
Any new bi-mode or DMU is probably going to be most versatile as a short/medium distance unit with a suburban door layout
I disagree there. I think the routes less likely to be electrified throughout are longer distance services like Cardiff-Portsmouth, which may cut across regions with electrification but traverse sections of track with no electrification. Also, there is a short-medium term prospect of suburban-door stock being cascaded in the form of Scotrail 170s and TPE 185s, but no large source of regional express units to boost capacity on regional express routes like Manchester-Swansea.
course you don't like and is probably not best suited to the Waterloo Exeter route but would be fine for TPE yes I know you would disagree on that but I use the North TPE frequently and the 185 arrangement is more suited to it.
I don't see that releasing Exeter 158's will really be enough to solve the DMU problem. As far the Waterloo Exeter route goes then the economics of bi-mode I imagine work best when a substancial part of the route is electrified which it currently isn't your also likely to need a Diesel/AC/DC bi-mode as even if AC electrification didn't get extended to Salisbury or further, it would need to be AC capable for possible pending electrification.
Of course the unit would need AC capabilty too, all 3rd rail units built since privatisation have had pantograph wells I think. If releasing SWT's diesel fleet isn't enough to solve the DMU shortage (and given some of us are/were convinced that the electrification cascades alone would be sufficient to avoid buying any new diesel stock) then the bi-mode (or should that be tri-mode) 175-alike units could also be ordered for Cardiff-Portsmouth as well, and/or possibly EMT's Liverpool-Norwich and/or the TPE south route which isn't yet earmarked for wires.
I would be interested to know what the issue regarding Waterloo capacity is/ is it a performance issue or lack of enough units.
In the article on the class 707s on page 11 of the Oct 2014 Modern Railways, it describes the 707s as 'Plan C'. 'Plan D' does not appear to be clearly defined yet, but it says
ETCS "would make 30tph possible, but that is only feasible with an all-electric fleet, implying electrification of the Salisbury route." I interpreted that as 'DMU performance isn't good enough' although that isn't there in black-and-white.
My own solution to Waterloo Exeter would be electrification to Salisbury, some of those early morning/evening Yeovil services which don't form an Exeter turn would be replaced by a shuttle to Salisbury, longer term might be a bi mode solution with electrification extension to Yeovil, or replacing the 158's with 220/221/222's
Without the Exeter services, would it be worth wiring to Salisbury even with Yeovil services reduced to a shuttle?