Just to add experience of this as of yesterday. Car has roughly 18k and was due to end in less than 2 months. Since all the cars I was looking at have been removed from the scheme or had big price increases I looked online at my account and it gave me the 2 year extension option so I took it. Certainly glad it was or I was thinking of leaving as electric is totally impossible for me.
You are lucky that your car was under the new limit for 2 year extensions of 20k miles. Ours has done 25k so they are telling me any extension is 6 months max.