The track base was 18mm ply which had been in use for 6 or 7 years.  I had unwisely used an own brand of preservative which, though reasonable, didn't last well.  The ply was beginning to separate and became very uneven.  I have replaced the bad sections and used 3 coats of Berratine which is oil based and soaks in nicely.  That was followed by 3 coats of Ronseal outdoor matt varnish.  (I will add more to the edges).