Could be an idea for those AV's nearing 12 years or even the current "loud" AX's - this taken from busandcoach.com

"Stagecoach is upgrading 50 midlife Dennis Tridents by replacing their Euro 3 Cummins ISBe engines with the latest Euro 5 versions. The conversion will cost £500,000 and will be carried out over 12 months. Stagecoach says the repowering will cut fuel consumption by up to 25 per cent, reduce carbon emissions and improve reliability. The upgraded buses will be spread across the group’s UK Bus businesses.The managing director of Stagecoach UK Bus, Les Warneford, explains: “Many of the vehicles are around 8-10 years old and are in good condition but require new engines. It makes sense to fit newer greener engines to pro-long their life.”

I wonder was this option ever looked at by the NTA. 200 buses would cost around €2.3million (or they could look at a few more if they wanted to) if they decided to do something like this in one year, plus maybe tighten a few of the pools and any lose ends and even update the seating plus fresh lick of paint and maybe clear the dmds, you'd have a bus that people would think was brand new apart from the number plate which most wouldnt really notice!

Also it could give a few extra jobs to any unemployed mechanics as well even for a few months if not a year!!