Dear Angela
1) the wood will last longer than that particularly if none is in
ground contact
2) to keep it sound you need a) to keep it dry b) to keep UV light
away from it
or
3) impregnate it with chemicals that protect it from fungi and/or keep
it water proof/ resisstant

If this were mine and I wanted long term protection I would
disassemble the cladding and take it to an OS borne double vac plant
that offered the wax water repellant as well as the fungicide and
treat the lot
I would bribe the company to leave it in the bottom of the plant for a
couple of days
take it out and dry it (say a few days)
prime every bit of end grain with Duiux Weathersheild green primer
I personally would then use a Sikkens or Saddolins stain to protect
from UV but if you do not want a stain put up with the UV degradation
that will occur
This will act also as a locus for precursor stainers that attack the
wood and prepare the way for true wood rotting fungi

If you simply don't want any of this aggro put on a spray application
of a water repellent each summer when it is dry

As for the product you rather liked, I note it is
a) not a preservative
b) is applied by a water-based solvent (and hence is not likely to get
into the side grain more than a small fraction of a mm
c) there is no such thing as a life-time gurarntee and anyone who
believes that has to be pretty gullible

I see no claims for it that can be compared with proper CEN, Agrement
or similar standards (eg from the USA equivalent) and when I see
products claiming non-toxicity I get suspicious (NO wood preservatives
are classified as toxic - they are generally only irritants)
Chris G

Thanks Chris. I don't think I would want to go to the extent of removing
it, I figured if I had to do that I would just replace it with Cedar! I
know it will turn silver and I am happy for it to go like that. I see what
you mean about a waterbased product, my neighbour in fact is contemplating
using decking oil, not sure if that is an option!

Angela