Sense check on Lease extension premium(s)

Hello Knowledgable people,

I'm in the process of getting a leasehold extension premium calculation done for an informal extension (I have not lived there for 2 years but freeholder seems willing to extend and I want to do it while i have money). I've rec'd a valuation report / calculation back from a valuer however I do not believe it to be accurate based on what i've read on this forum and around the web. Before I dispute it, I just wanted to get a sense check....

The property was very recently purchased for 475k and has 87 years to run on lease. Ground rent schedule is 3 x £50, 33 x £100, 33 x £200 and then it looks like the final 18 years is also £200. The flat is quite close to central london.

I requested the valuer provide me with estimates for a .) 90 year ext @ peppercorn b.) 40 year ext with same ground rent schedule as present.

In both cases valuer has used 485k as a value for the flat(which seems ok I guess).

a.) Valuer indicates total premium of £9,974.50 where reversion is £5,189.50 and Ground Rent payout is £4,785. To me, the groud rent looks way to high and the reversion looks to low...... was thiknig total would be more like 8.5kish
b.) Valuer indicates total premium of £6,935.50 (all reversion). To me this looks too high...was thinking closer to 6k

What do you guys think?

Thanks