The Regal was continuously improved upon with the Mk 4 in 1958, Mk 5 in 1959 and the Mk 6 in 1960. In 1962 a major step forward was taken with the Regal 3/25. This had a unitary construction body of re-enforced glass fibre. Polyester was moulded in two major units (outer and inner) and then bonded together and bolted to a steel chassis. A year later Reliant developed their own 600cc overhead valve unit which was Britain's first flowline production light alloy motor engine. In 1969 the 750cc engine was introduced and Reliant's production was such that it in 1969 the fifty thousandth 3/25 model was delivered. In 1970 the Regal 21E was introduced with 21 extras fitted at the factory.

It perhaps almost impossible to not mention the Reliant Regal Supervan III without someone mentioning “Del Boy” in the BBC series “Only Fools and Horses”. The series featured a yellow van that was used by the main characters. Such is the power of this classic comedy series that many Reliant Regal Supervans are being sold and purchased at over inflated prices so that people can create their own “Del Boy” van. In one way this is good as it is keeping these Reliants maintained and on the road ... on the downside within a few years every Supervan could be a dirty yellow colour!