Do you know if the 707s ever planned to be a full fleet replacement at some point, and how come the order for 36/40 didn't happen?
Looking back there seemed to be a fair bit of smoke and mirrors in play, in essence a lot of dealings well above my pay grade that I wasn't party to, but AFAIK there was never any plan for a one-off suburban type (as in 701) at that point in time. The issue SWT/DfT had at that time - I am talking 2007/8 here - was a massive uplift in demand that had become embarrassing on the Windsor side routes.
The idea behind how the entire (original) 10 car plan was to be resolved/implemented was the brainchild of someone for whom I have the utmost respect - always will have - but with public money (via DfT) involved there were limits as to how this could be done. I first became directly involved in late 2007/early 2008, a point at which plans for alterations to daily diagrams as referred to above (460 initial input to get the rebuild program rolling) and the associated cascade(s) planned, as in x number of 458/5 would be in service by x date etc, and on which exact peak services - had to be written in to the various contracts.
Given events outside the industry (in The City etc) in 2008, the industry nationwide was hit with a double wammy. In SWTs area early plans for an uplift of service provision (WLs) were ongoing, but then a need to reduce realtime costs, and reduce them vary fast. This was acheived by and large, with minimal impact to the OVERALL train service.
After the initial 458 conversion plan, there followed two further Add-on's, in order to increase the number of 8 car formations that could be increased to 10 cars, as it was now seen that the main suburban side was also in dire need of increased capacity, and in truth, this far down the line, I can not now remember if the 'new build' scenario (Desiro City as the then theoretical units were referred as) or the 456 side of things came first. I think it was the DC plan, the add-on 456 following on, but I may be wrong. Although I still have my initial draft working diagrams for all of this (458/5 dated 2008, 707 dated 2013), it's not relevant now, as the 458/5, 707 & 455/6 workings that became reality were as per the original draft workings, all bar a few minor changes, so there's nothing new to be added to the history of the whole plan as was.
As for the decision to go for 30 x 707 as opposed to the other two options, I suspect the usual suspect (in DfT terms) was the reason......the ££££...! In operating terms that was why 3 x 8 car 450 diagrams remained on the Reading route. I suspect it was a business commercial v cost decision and rightly so when public money is involved, especially given events of 2008 etc.