Is this not just a case of preventative v reactive maintenance?

As a now former Deptford employee, I can say that the engineers do try there best with a limited budget. Also the Citaros were very thirsty and parts were very expensive. I was told Mercedes wanted £7,000 for an exhaust and £20,000 for an engine (that's when the second hand examples started dying and they were 07 plates). Realistically, the actual vehicle probably wasn't worth that much.

Also, when you have a yard full of buses the same age, they would tend to develop faults and failures around the same time whereas with some newer vehicles in the mix it may not be as noticeable.

Even so, second hand (my guess) is the way to go rather than buy new unless government grants are available (electric or alternative fuels). Fleet and depot standardisation should also help reduce costs.