Thanks for the reply Steve.
Your are correct the 230e has no catalyst and the 300se does, both these cars are European models running in the UK. What sort of 'real time' tests can be performed?
The 300se is running pefectly except it will sometimes hesitate when accelerating hard especially around 2000 -> 4000rpm. This seems slightly worse when the engine is cold. Plugs and distributor cap + rotor have been replaced. New fuel filter going in next week.