How to add an extra gear to your 4 (or 12) speed Brompton

In this video we go over how to add an extra gear to your Brompton. This should work on any Brompton with the Advance derailleur system (T/P Lines).

Parts list,

-Shimano 10 speed mountain bike shifter. Any series will work but it can not have the optical display.
-11 speed chain, any chain will work as long as it is desighned to work with the Shimano cassette.
-11 speed Shimano cassette. You need the smallest 5 cogs from this. I used the Deore CS-M5100 11-51 cassette.
-Shift cable inner end end cap, these should come with the shifter.

Tools list,

-Cassette lockring tool, any will work as long as it is Shimano compatible. you may also need a big adjustable spanner if your tool does not come with a handle.
-Chain breaker. Any brand/type should work.
-Allen key set. Nothing fancy, as long as it has 2mm, 3mm, 4mm, 5mm etc its good to go.
-Chainwhip. There are a few tools designed to hold the cassette when you are removing the locking. Any style should be fine.
-Cable cutters. A good pair is nice to have but as you see in the video anything works.

FAQ
I will update this as questions come in