Value of property low

Hello

I have a leasehold flat (England UK) , which previously had 79 years lease remaining; and I paid to extend it a few years ago, it’s now near to 180 years, but I’ve noticed the sale value was low before the extension; and it’s still the same low figure even after extending.

Should the long lease not have an impact on the value, rising it to a substantial amount?

I’m looking to sell up basically; and local agents are very clueless to this matter!

Thanks