Little bit more information now I have found the books I lost (dont ask)
Vauxhall cars 1945-1964page 36:
"September 1963 was to see Vauxhall launch its first small saloon car in nearly 30 years, with the introduction of the Vauxhall Viva HA. ........ Luton turned out the 2-millionth Vauxhall, and there was literally no capacity for a small car.
To address the problem of capacity, and given the incentive of selective financial assistance, the company announced it was to open a new plant at Hooton, Near Ellesmere Port....... and it was decided that it would be the base for the manufacture of the new 'H' Concept car. Construction began in August 1960, and the first Viva rolled off the specially-built Ellesmere Port in August 1964. However, the success of the small BMC and Ford saloons (especially the Mini), prompted Vauxhall to commence the production of some Viva Saloons at Luton in September 1963, just in time for the forthcoming motor show."
Body shells were produced prior of these dates, indeed shell production at Ellesmere port was up and running very successfully prior to August 1964, these body's were shipped by train to Luton for assembly into complete cars.
Conversely once the production line was up and running at Ellesemere Port but manufacture of its own running gear was still being developed, running gear from Luton used. Then on 1st June 1964 first fully assembled car using its own parts was assembled. This is the photo we see in one of the earlier posts.
Its easy to get all mixed up with dates, But 1963 was the first year of production not 1964. the exact date varies from production line to production line...
Below for you super anoraks is a prototype car (note the front end and rear windows) being tested circa 1961-2 at Chaul End test track, Bedfordshire.