I think this number is probably correct - I was down there a few weeks ago and was very surprised to find none at Howletts and just a single enclosure at Port Lympne. It seems only a few years ago that there several groups between the two parks.
There used to be Siamangs in at least one, possibly two of the old 'wood & mesh' enclosures in the woodland walk behind the mansion at Howletts. Nowadays most of those cages seem semi- derelict- they're certainly a far cry from the excellent extensive primate enclosures they are doing nowdays. I only wish some more could be built for the other groups of Colobus, Drills, Dianas and other primate species, a number of which still live in the old-style cages.