You might be able to reduce the CO2 emissions on your car, but it won't change the road tax. The tax is based on homologated manufacturers declared emissions at the date of buildas shown on your registration document. Likewise if you subsequently increase the CO2 emissions (performance chip, tyres etc) it won't increase the tax.
The difference between the two models is likely to be a combination of tune; gear ratios; and low rolling resistance tyres.
There are two engine choices for the 1.6crdi. a high output - 113bhp and a low output - 89bhp. Yours will be the high output engine usually found on (some) gs models and all ls models.
The lower engine qualifies for the £35 p/year tax.
Hopefully ALL manufacturers will work towards the lower end of the scale (ford now produce a £0 tax band diesel!) not only to 'save the planet' but to stop the rob-dogs that run this country from creaming even more money of us poor motorists.
Nothing you can do to reduce your car road tax price. All road tax prices are based on the emissions C02 rating from your car. Two cars with the same engines, yet manufactured in different years, might have different ratings simply because the manufacturer has tweeked the engine.
It is all based on the emission figure shown on a vehicle's log book - no matter what changes you make to your car the road tax price will always be based on that figure on the log book.
The only way to pay less road tax is to buy a newer car with a lower official C02 figure. Exactly why I got fed up with my Mercedes C180K auto which was £260 per year and traded it in for my Kia C'eed.. £30 per year