I am 4.5yrs plus in my lease and was told I couldn’t keep longer than 5yrs. My only problem and why I wouldn’t extend again, is if you keep it for full 2yrs extension you only get £250 additional GCB. Or in my case because my new vehicle will arrive next month I will only get an additional £100 GCB rather than on a pro rata basis, which I was expecting. Yet you still have to pay your full award to MB during the extended lease.
So from my personal view it’s not cost effective and a big no no.