A number of Optare Vectas at Arriva North East were withdrawn for years before many had their MAN engines replaced with Cummins Units and re-entered service.

The case with 4719 is that it has been used as a Christmas Tree, instead of having say 5 buses off with 5 different faults. They take all the required functioning bits of one of the vehicles (probably the one that has the longest wait time for a part), and distribute them amongst the other 4 vehicles, so instead of 5 buses off, you only have one bus off the road longer, whilst you have 4 available for use.

It's common practise with a lot of operators, buses, trains, metros, trams, haulage industry. They all do it.