Wasps effectively paid £18m for the short lease when they took on ACL. That was the value in the accounts of ACL.
At 31/05/2014 the ACL balance sheet showed net assets of 6.8m in the accounts to 2015 ACL made losses which would have decreased the Net asset position. Wasps paid 5.5m plus some add ons for ACL
Strutt & Parker valued the 250 year lease in April 2015, and by that time ACL had a long term lease in place from Wasps Rugby which adds value to the asset held by ACL
Wasps bought ACL not the lease in October 2014 - that's all assets less all liabilities of ACL
The lease extension for 250 years came in to being 15 January 2015. It could only be extended to the benefit of the current owner of the lease ACL, whilst ACL existed there was no open market to sell it to (for at least 39 years)
The lease is charged against the bonds issued May 2015