Come to think of it, the majority of those buses that were withdrawn earlier than expected due to an accident or whatever would've been replaced, the replacement contract (Mercedes-Benz 1999-2011) and (Volvo 2011-2016) updates and replaces ageing vehicles anyway, so if a bus does have an accident and needs to be replaced, a simple switch with the replacement plan with new deliveries means it'll get replaced, not necessarily immediately, but within a given time frame. Of course it has been a different story with artics (remember the period between 93-99 where we got no new buses, they obviously couldn't replace artics that were withdrawn). The past decade, there may have been a 1:1.5 ratio with artics being replaced. I'm not 100% sure. Here's hoping that the Volvo contract will deliver more artics than needing replacement.
2014 was sent to the Morley Contract (as oppose to being destined for the Marmion Contract) to replace 1099. The Mercedes-Benz contract would've replaced those Reno's and other buses which were deemed repairable, they can't just keep on losing buses without replacing, it'll cause a logistical problem.
Btw, the gas tanks have never exploded on any of our CNG OC buses, like I said earlier, the engine bay may catch fire, but due to safety measures and etc. it would be difficult for a CNG tank to explode, those incidents involving about 4 or 5 buses over the past few years had nothing to do with the CNG tanks and never caused such thing to 'blow up' as you say. But more related to the running components (i.e. engine), something which has happened around the world with Mercedes-Benz buses and is not an entire issue relating to CNG. Because diesel buses have caught fire too!
Re: 1610, I don't remember exactly, but I think it was in 2008/09? Herbert should be able to clarify its withdrawal date.
Re: 599, at this stage, unless there is a change to the buses expected for delivery before July, then I'd assume 599 will still be going ahead with repairs. I also recall this bus being withdrawn about 2 years ago aswell, but was placed back in service not long after.