If an occupational pension scheme held shares in a sponsoring employer before 6 April 2006 the shares can continue to be held.
If however, the scheme buys shares in a sponsoring employer after 6 April 2006 the 5% limit will apply and the total value of shares including the value of the shares held prior to 6 April 2006 will have to be taken into account.
HMRC Pensions Tax Manual - PTM122000: Investments: shares and equities