Mechanically and electrically they are just iterations of previous build and they are well versed with the bodywork construction method. Also I wouldn't have thought Hitachi will be as daft as Bombardier to rehash the base software of the TMS like they did with 701s. Lets hope there is more transparency about what is going on as all this delay is costing money and hardly helps deal with the capacity issues MML has.
No they are not just iterations of previous builds.
Firstly this is the first body to be welded at Newton Aycliffe. And it is taking longer than expected per vehicle to complete.
Different length of vehicle means a whole redesign of everything mechanically and electrically.
New type of engine, and more engines per 5 car train than all other 8Xx models.
Due to different length of vehicles all testing of the complete train needs to be redone, as different length, different weight all changed how train performs.
Different length train means different pantograph testing.
The above are just a few of the things I can think of, but read any article about the train and it is basically a new train and can not be compared to any previous 8Xx classes.