Keith
Had a look at the photos, bit had to tell as the deck is wet, but from the handrail, look's like a stain not varnish and most probably Ronseal, s cleaning alone won't remove all the stain it is going to need stripping back to bare wood. Can you find out what was used?
Phil,
Had a quick look at that product, reading the page it sounds/looks like a per-carb but without seeing the MSDS can't really say. Per-carbs are ok-ish for light cleaning and that's about it really, big in the States for home-owners to use, as they can't do much damage.