I am looking to replace cavity trays on my house where it's suspected that the tray (bitumen I think) has failed.

 

I am looking to use 450mm type E trays, which, you only need to take 3 bricks out at a time to fit.

 

The question I have house far over the window do they need to go. E.g a 900mm window, does the length of cavity tray need to be min 100mm longer either side (1100mm total)

150mm is the norm but tend to go up to a brick further. 

I did them on one of our rentals and hired a brick saw for the day from Smiths rather than using an angle grinder Cuts through like butter 

Less mess and much quicker and neater 

You could try just painting the wall above with StormDry, which is supposed to last 25 years.

Those saws are supposed to be the mutts nuts for patching