The fact is FGW^▸ (First Great Western) allow for a certain percentage of trains/diagrams to be covered each day on overtime /rest day work when they calculate the level of staffing required. This is fraught with danger as it assumes that staff are available to work their days off, that they are at the right locations to cover the vacant turns, they are on the correct shifts and have the correct rest periods to cover these turns. Also the constraints of the Working Time Directive apply as to the maximum working week.
In some cases staff are rostered up to 44 hours in any given week and if they work a 10 hour Sunday and a rest day this means they are working in excess of 60 hours in that week. Over a 17 week period they are not permitted to work more than an average of 48 hours and cannot work more than 13 days in any 14 day period so at least one Sunday/Rest Day every 2 weeks they are not available to work.

Also not all staff are willing to work overtime which is their right so the pool of staff available to cover these vacant turns is greatly reduced. 

The problem has been that during the last 12 months or so of the previous franchise no vacancies were filled as until a company knows that it is going to win the franchise they will not pay training costs for staff that might not be theirs in a years time. Also FGW^▸ (First Great Western) base their staffing numbers around different formulas than BR^▸ (British Rail(ways)) used to do and FGW allow for a certain percentage of turns to be covered by overtime which at peak times of the year Xmas/New Year, Easter, weekends and July/August can be very dangerous.
BR used to base their establishment figures around the Summer Timetable which was when more trains tended to be run and carry the surplus the rest of the year.
Also staff morale is a major issue with staff no longer being prepared to work large amounts of overtime.

I am led to believe from staff at FGW^▸ (First Great Western) that the reason that the front of the train is the portion that is platformed is that a consistent stopping position was agreed with the HMRI^▸ (Her Majesty's Railway Inspectorate). If there is a signal associated with the platform there are going to be occasions when this is showing red and therefore it is not possible to go past with the First Class and put the rear on the platform. Also the Train Manager now has to be at a particular door panel for each short platform and if in the above scenario they positioned themselves towards the front to release the doors to the rear of their position and the signal was at red they would not be able to release to the rear as some of the rear coaches would be off the platform and would then only be able to release those doors in front of them. On a platform which should hold 6 coaches this would result in only being able to open the front 2.
Not all platforms are fitted with boards to indicate exactly where the driver should position the train if the rear portion is to be platformed and finally all the wheelchair positions are now located in one half of the train - coaches E and G and it is far easier and quicker to reposition a train by drawing forward than by asking permission to set back where the driver would have to change ends, set back into the station and then change ends again.