HMRC and share valuations

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HMRC like to apply discounts to share valuations due to perceived lack of control of the taxpayer, where the shares held are less than say 50%. this does seem ludicrous as family businesses tend to be bought and sold as one rather than in part.

the issue is around a trust that acquired some shares in the early 1990s, then disposed of the shares a few years ago. The shares held by trust were sold along with the remaining shares held by the family members i.e. the company was sold in its entirety.

HMRC have argued that the early 1990s valuation should be subject to a discount because the trust held less than 20%, despite the trust and the family members directly owning 100% of the share capital.

I've looked at the statutory hypothesis (IRC v Gray) and it makes no mention of applying a discount, only that buyers and sellers need to be at arms length.

i'm after thoughts on how to approach this?