Its major advantage over the Leyland Atlantean was that its patented concentric-drive gearbox enabled fitment as standard of a dropped-centre rear axle, allowing a body suitable for low bridges of 13 ft 5 in (4.09 m) high to have a centre-gangway seating plan for the full length of both decks.

In comparison, the low-height Atlantean needed an awkward side-gangway abaft upstairs to allow legal internal headroom throughout.

Instead of the very low horizontal frame of the first two chassis, this was slightly higher at the front and ramped gently upward and tapered outward toward the rear, where the Cummins VIM V6-200 engine was mounted.

This 9.6-litre 90-degree V6 engine developed 192 bhp at 2600 rpm and was compact enough to fit under the rear seat on a bodied bus and allow drive to pass through a conventional Daimler SCG Daimatic four-speed semiautomatic gearbox to a straight spiral-bevel rear axle manufactured by Eaton, an electrically operated two-speed axle being optional.

A left-hand drive chassis and Duple Coachbuilders 51 seat coach demonstrator were shown at the 1964 show together with the first example sold to an operator, 6000EH of Potteries Motor Traction (PMT).

The PMT bus had a 50-seat Marshall body to British Electric Traction style, featuring no steps and a slightly ramped floor up to the rear bench.

This bus entered service in late 1964 and unprecedentedly the union branch operating it were so impressed, they sent a letter of congratulation to Daimler.

Early UK Roadliner customers were corporations in Wolverhampton and Sunderland, independents West Riding Automobile Company, Wakefield, and AA Motors, Ayr.

By the end of production in 1972, 333 Roadliners had been built; besides the UK, examples had gone to Australia, Belgium, Canada, Poland, South Africa, Spain, and Switzerland.

Secondly, drivers newly allocated the type had not driven a vehicle of equivalent power before, and most were not trained in the use of the semi-automatic gearbox, which lacked the engine-braking effect of a "solid" transmission.

This resulted in many engine losses due to overspeeding; no easy way to control this was found, as the pure fluid flywheel form of transmission was used rather than a Leyland SCG-style lock-up clutch.

In 1966, Daimler launched a single-decker variant of the Fleetline and thereafter marketed the SRP8 as a coach chassis in the UK, the final demonstrator had a Plaxton semicoach body and was sold to City of Oxford Motor Service to work London expresses.

Prior to the BLMC merger, Sir William Lyons had commissioned an all-alloy 6.7-litre V8 turbodiesel engine from Rolls-Royce; it was derived from the petrol engine used in the Silver Shadow car, examples were on test by the time British Leyland had been formed, but the project was cancelled on the orders of BLMC chief executive Lord Stokes.