Mark,
According to the 'bible', Brian Lavery's 'Nelson's Navy', and the diagrams, both the Armstrong pattern guns (approximately 1720 to the American War of independence, although some were in service until about 1808) and the later Blomefield pattern (1780's to Trafalgar and beyond) had a 'first reinforce' ring approximately mid-way between the trunnions and the breech, which would appear to be missing here or is somehow hidden.
The former also had a 'chase astragel' ring just forward of the 'second reinforce' ring near the trunnions. This would also appear to be missing. Both guns had a 'muzzle astragel' ring, set back from the er, muzzle, the Armstrong gun appearing to have it set further 'aft' then the Blomefield version – and much as in your photograph. However, I am not convinced this is an older Armstrong pattern, and I don't think it can be a later Blomefield gun either, since the tell-tale loop to take the breeching rope, is not present.
Therefore, and unless I am completely wrong, I am surmising that these are indeed two of the captured French guns, and which were originally set around Nelson's statue. However, to date I haven't found any diagrams of French cannon to check whether this is likely. When the owners get the guns scanned, it might be a good idea if they checked the ends of the trunnions. This is where any markings are likely to be, certainly on British guns.
Nice looking carriages, by the way, even down to the different sized trucks.
