Although the Rover 75 has a hi-tech common rail turbodiesel, the 25 and 45 and the MG ZR use a conventional turbodiesel by Land Rover and supplied through an agreement with Ford.
The company's main rivals in the lower-medium and supermini sector offer a broader range of diesel engines, many using common rail technology.
Having a modern diesel engine option has also become crucial to attracting fleet sales under the new carbon dioxide-based company car tax rules .
John Parkinson, director of sales and marketing, said: 'We believe our diesel engine can be further refined, but over time we will develop and have access to a wider range of engines.'
He added that although it would be another two years before the next new medium car from Rover, the company would concentrate on 'niche' features to maintain the current level of interest in its products.
He said: 'Our performance in the UK has been very good, in line with and sometimes exceeding our business plan.
'To ensure we can maintain the level of interest in our cars we will continue to add features like the Monogram programme which allows customers to tailor cars to their own specifications, and create niches within the range.
'We are reasonably confident that we can continue to meet the expectations in our business plan on that basis.'
Parkinson added that MG Rover was seeking new relationships with potential fleet customers and the new MG products had helped raise its profile.
He said: 'We have set up a programme to expand our database, and the MG cars have helped raise the profile of Rover, where people know about the MG products but are not necessarily aware of the full Rover range.'
The move was announced at the unveiling of a new Rover concept car at the show which indicates the future design direction for the company. The Rover TCV (Tourer Concept Vehicle) has been designed for production feasibility and comes as the British company is on the verge of entering a partnership with China Brilliance to develop future products and open up new markets.
The first all-new Rover car since the 75 is due to go on sale in 2004, and could be similar to the TCV (pictured), which has an adaptable seating plan with folding rear and front passenger seats. It also features a hinged lower bumper section at the rear which reveals a large box-shaped under-floor storage compartment.
MG Rover design director Peter Stevens said: 'The work we have already completed with MG has given us a better understanding of what Rover stands for. It is the epitome of British design.'