Readers' forum: Client list

My corporate client has sold part of its trade that is a financial advisory business. The goodwill of that part of the business consists of a client list and associated commission stream and this was sold for £500 000. The total goodwill is valued at £1.5m. Part of it was purchased in 2012 for £100 000 qualifying for tax relief on amortisation (current net book value (NBV) £50 000). And a further £500 000 was paid for goodwill in 2016 and treated as a capital gains asset (NBV £400 000). In addition the company has won new clients at no cost.

I am at a loss to know how to calculate the capital gain and intangible credit. Is it simply by reference to relative values or do I look at the lists of individual clients...