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Good day, I was driving home from work today and started to smell a burning smell, like a mix of rubber and diesel, also notice that the engine sounded louder/rougher too.. Drove the rest of the way without trying to put much stress on the engine.. When I got home I opened the bonnet and sure enough something sounded amiss, when I turned off the ignition the car it almost shuddered to switch off and then upon restart the engine it sounded like it was purring? Looking closer you can see the timing belt is changing speed each time the pur happens, any ideas guys? The car is a 2011 vauxhall astra 1.7 Sri diesel, with 77k on the clock

havent got a clue

but ------ service history other levels off unusual like squeaks 'lack off power 'vibration' unusual lights illuminated or extra fuel 'oil ' water consumed than usual

Overheat smells and shudder stop is just typical of an OVERHEAT. Don't know the model but timing belts are seldom visible under a bonnet without some cover removed!
In general suggest you are seeing a pulley belt slip due loose or one of the following semi seizing : a water pump, alternator, power steering, mechanical driven fan or air conditioning compressor.
Usually there is a squealing noise as well.

I doubt it is the timing belt slipping you are probably looking at an auxiliary drive belt for the alternator or air-con compressor. If the timing belt had slipped it would not run. It could be that this belt is slipping and the burning smell is the rubber belt heating through slipping friction, and the noise is the slip. Focus on that first, it might just need the alternator bracket adjusted. Hard to say blind.

DWD

Doesn't sound like the belt is slipping at all. Sounds to me like it's just matching engine speed and if the engine is hunting then the belt will follow along.

Without further information it's impossible to tell what the noise/smell could be. It's just gonna be a case of taking it to a garage or giving some further details or a video of the fault.