^^^^^^^^^^Ramone and ^^^^^^^^^^^^^ Robert
The Swift was a rear engined single deck stage carriage design with the AEC AV505 engine. The Merlin was basically the same bus but with the AV691 at the behest of London Transport who bought most of those built. The AV505 Swift was the better one of the two. The Merlin was found wanting in London traffic conditions and suffered from over heating problems. They were often seen at the side of the road steaming well with a pool of coolant underneath.
The Leyland National was built at the new Leyland bus factory at Workington and it had the headless wonder Leyland 500 series engine (not sure which variant but at a guess the 510). They were noisy - sounded like a bag of nails jumbled up with nuts and bolts - smokey and unreliable. Enough said!