I have just tried Chef brown sauce from Tesco. It was not in the Freefrom section but in the Irish foods so it is not a premium price. It is labelled as gluten free and tastes a lot like the good old HP! Haven't heard it mentioned here, sorry if it has. Just had a bacon white roll (white roll also from Tesco) with brown Sauce and brought back happy taste memories.
I have tried other sauces but this is the closest I have found to what I remember. Only went in for the 3 for 2 offers on Free From foods so it was a bonus find.