The London Stadium concessionaire agreement was signed by West Ham and LLDC in March 2013 which includes clauses within the contract that say 12.5% must be paid if the club is sold between £200m and £300m in the last five years of the ten year anniversary of the contract commencement date with 2o% due on sums over £300m. Banks and shareholder loans can be subtracted from the tax calculation.
If the club was sold for £350m they would be able to exclude the £45m owed to Sullivan and Gold and £10m owed to Tripp Smith in shareholder loans. The short term bank debt of £30m would also most likely qualify for exclusion under the contract.
The qualifying amount would, therefore, be reduced to £265m in which case the 12.5% tax would come into effect for £65m costing the shareholders £8.1m
Split out by shareholders Sullivan would pay around £4m of that, Gold around £3m, Smith around £800,000 and Brown and Harris family around £325,000.
The reason I have chosen the £350m figure is that is the valuation Mike Ashley has put on Newcastle United which remains unsold since he announced the club for available for sale in 2017.