.We fitted a new staircase 2yrs ago (pine) we used a clear varnish on it and it is now going yellow which isnt acceptable to us now Can anybody advise us as to what we could use to change the colour without stripping the staircase.
nothing, frankly. if you want that white pine look then you will have to rip out the enitre staircase and refit. it isn't the varnish that has yellowed but the wood itself.
pine, as with all types of wood, discolours with age as the cells in the wood begin to break down and decompose. this does not in any way affect the structural integrity of your staircase but once it's yellowed that's it.
think about how paper yellows over time - paper is almost always made from the pulp of pine trees. the decay of the cells is caused by acids that naturally occur in the wood.
you could try sanding down to expose fresh wood below the surface but its only a matter of time before that yellows too (although wood will generally discolour all the way through at a fairly steady rate) i fear that if its disagreeable to you then your only option is to paint it white.
also if you do decide to sand it down, you could try a swedish product called "lye" which has a mild bleaching effect giving a somewhat "beach house" look [ if that suits your needs]. After use it should be sealed, but there is a specialist non-yellowing product which just cannot remember the name of at present. Suggest you google "lye" to find suppliers in your area.