Am I right in thinking that there were about 4 loco's all carried the name 'Flying Scotsman' at the same type as each engine was upgraded. I would have thought they would need a handful to maintain a proper service before diesel took over completely.

It was the London and North Eastern which, in 1924, officially renamed the 10:00 Special Scotch Express linking Edinburgh and London in both directions as the Flying Scotsman, its unofficial name since the 1870s. To further publicise the train, a recently-built Gresley A1 Class locomotive was named after the train, and put on display at the 1924 British Empire Exhibition.
Due to a long-standing agreement between the competing West and East coast routes since the famous railway races of 1888 and 1895, speeds of the Scotch expresses were limited, the time for the 392 miles (631 km) between the capitals being a pedestrian 8 hours 15 minutes. However, following valve gear modifications, the A1 locomotive's coal consumption was drastically reduced and it was thus found possible to run the service non-stop with a heavy train on one tender full of coal. Ten locomotives of classes A1 and A3, which were to be used on the service, were provided with corridor tenders; these avoided engine crew fatigue by enabling a replacement driver and fireman to take over halfway without stopping the train.

The London and North Eastern Railway LNER Gresley Classes A1 and A3 locomotives represented two distinct stages in the history of the British 4-6-2 "Pacific" steam locomotives designed by Nigel Gresley. They were designed for main line passenger services, initially on the Great Northern Railway (GNR), a constituent company of the London and North Eastern Railway after the amalgamation of 1923, for which they became a standard design. The change in class designation to A3 reflected the fitting to the same chassis of a higher pressure boiler with a greater superheating surface and a small reduction in cylinder diameter, leading to an increase in locomotive weight. Eventually all of the A1 locomotives were rebuilt to A3 specifications.
The names for the locomotives came from a variety of sources. The first, Great Northern, was named after its parent company. Others were given the names of high-ranking railway officials, but most were given the names of famous racehorses. One was named after the company's most famous long-distance passenger train, the Flying Scotsman. Flying Scotsman is the sole survivor of the class to be preserved. It is currently under mechanical overhaul at the National Railway Museum.

The LNER Class A3 Pacific steam locomotive No. 4472 Flying Scotsman (originally No. 1472) was built in 1923 for the London and North Eastern Railway (LNER) at Doncaster Works to a design of H.N. Gresley. It was employed on long-distance express trains on the LNER and its successors, British Railways Eastern and North-Eastern Regions, notably on the 10am London to Edinburgh Flying Scotsman train service after which it was named. Flying Scotsman is regarded as the world's most famous steam locomotive.[1]
The locomotive is notable for having set two world records for steam traction; becoming the first steam locomotive to be officially authenticated at reaching 100 miles per hour (160.9 km/h) on 30 November 1934,[2] and then setting a record for the longest non-stop run by a steam locomotive when it ran 422 miles (679 km) on 8 August 1989.[3]
Retired from regular service in 1963 after covering 2,076,000 miles (3,341,000 km),[2][4][5] Flying Scotsman gained considerable fame in preservation under the ownership of Alan Pegler, William McAlpine, Tony Marchington and finally the National Railway Museum. As well as hauling enthusiast specials in the United Kingdom, the locomotive toured extensively in the United States (from 1969 to 1973) and Australia (from 1988 to 1989).

We used to climb the advertising board structures at the side of Hayes bridge years ago, taking train numbers as they went steaming by, I think the object was to write down the numbers and tick them off as when we saw the same trains, at least it got us out, so many classes, we knew them all, remember Brian.

What was that book? Had all the train numbers in it, underlined each number when you "spotted it"

I think the one I used was Ian Allen's Loco Spotter.