So it's been over a year in the garden now so I thought I'd see what has changed in that time.
Here is a recent picture and it still looks almost as new.
Not all perfect though
A split has developed in one edge piece. The wood is resinous and has filled its own crack. Maybe the resin made the crack?
Only a few (maybe 4 or 5) pieces seem to have excess resin the other 99% of the table has no problems. The resin exudes through the polyvine varnish whilst apparently doing no harm to the finish. It leaves clear resin crystal on the surface both end grain and faces. Fortunately it wipes off with a few rubs with a rag with white spirit and it seems set not sticky.
I've not tried wiping in the gaps so the resin crystals are obvious there - if you are looking for them. This wood was someone's 20 year old flooring from ebay so the fresh resin is surprising but not really a problem.
The Polyvine extreme varnish is still impressing me. I used it on a garden bench that was not covered over winter (the table was) and that is as new too.