Best to speak to this organisation, which being Government-funded should provide you impartial advice. It will not provide you a valuation for your proposed lease extension and nor can I - you need to take your own professional advice over that matter.
http://www.lease-advice.org/
However just because a HA says that they have received a valuation doesn't mean it is correct - indeed since the valuer is working for the HA it is more likely to be on the high side.
There is always room for negotiation in these matters.
If you cannot agree, there is a process using a Tribunal to determine an outcome. Lease Advice can help you with how it works.
BM