The stake, which would cost Stratagem pounds 1.5m if it was to purchase it, is being described as an investment.
But close followers of Harrison say it could lead to a bid from Stratagem or a change of management with Stratagem's Bernard Kerrison taking an active role at Harrison.
Mr Giddings joined Harrison when Antares, the group that owned the Covent Garden General Store, reversed into Harrison in 1991. At that time Harrison shares were valued at 115p. They closed yesterday at 4p. The group has turnover of more than pounds 50m a year and made a loss of pounds 455,000 in the six months to September.