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I'm considering buying a 1998 Volvo S40. It has a rough idle particularly when aircon and lights are on. I realise it is hard to diagnose without viewing the vehicle but are you able to make some suggestions on what can cause the rough idle.

We're not aware of any particular issue with this model, but as a generalisation, poor idle when the a/c is on could relate to an electrical problem. When the a/c is switched on, it draws power from the engine, so there is a system that increases the idle speed to compensate for the power draw which reduces idle speed. Some models have an electrical solenoid that raises the idle speed when the a/c is turned on. It could be a problem with the solenoid or the signal to the solenoid, which usually comes from the engine management ECU.
Other possibilities include general tuning (eg; spark plugs, leads, fuel injection system, an air leak etc).

It turns out that it i a misfire. I have had the spark plugs replaced and the garage mentioned that the resistance on the HT leads was a bit higher than it should be. The musfire is still present, could the HT leads be contributing to this or is more likely something else?

High resistance in the leads could be a contributing factor. As is often the case, it's important to get the basics right before any further diagnosis takes place. Plugs, leads and the ignition system in general are all important.