Building materials group Grafton has continued to build its stake in its smaller rival Heiton. Grafton disclosed yesterday that it bought another 383,000 Heiton shares on Thursday to take its stake to 23.3 per cent.Some observers believe Grafton is gearing up for a full-scale bid for Heiton while others believe it is stakebuilding to prevent a takeover of Heitons by one of the big British merchanting groups.