I have to guess here that this is a 1.3 diesel with a diesel particulate filter. These have been a bit troublesome in small Vauxhalls and Fiats fitted with this engine when the active regeneration of the DPF is not completing. It might help to switch to higher cetane diesel such as Shell V-Power or BP Ultimate. It is also true that the closer to an oil service and the more contaminated the engine oil the more likely you are to suffer DPF trouble. (Because its Galaxys are used a lot in heavy traffic, Addison Lee changes the engine oil every 5000 miles to prevent this.)