@Andy For evey completed year of extension after the initial 3 year lease the GCB is as follows.
For every completed year after 3yrs GCB is
3 years £600
4 years £700
5 years £900
As you can see after the 3yr lease the GCB is not paid at £20o per year as it is with you initial lease. If you were to keep your 3yrs 11months you will not get any additional GCB. Likewise if you kept your car under 5yrs but over 4yrs you will only receive £700. You still need to pay your full allowance if you extend, making a 5yr lease cost £16k+ of benifit payments.
The old £250 gcb worked as above and expect the new gcp to be the same.
I extended with my last car because of the initial lockdown. Personally I woud never extend again. However we are all different and have different reasons that suit are individual needs. “Whats good for one may not be for another”