My car is an Audi a3 2.0 tdi

every time the start:stop kicks in it struggles to start and flashes up with the coil light on the dash. I had the battery changed about 6 months ago

Hello Danny,

When you say the coil light flashes up - do you mean the glow plug light? 
If so, there a place for you to start. 
I think it’s pretty clear that you will need to get the car plugged in to diagnose the issue, since it will be virtually impossible to remote diagnose where the root cause may be. 
Perhaps you could let us know how you get on. 
Kind regards,

Gareth. 

Yes the glow plug light. I have had the car for 8 years and I don’t think I have ever had them changed. What do you think?

Thanks Danny,

If it were mine, I would not be jumping to any non-tested assumptions. All of this maybe due to something as simple as a poor connection rather than the glow plugs themselves. I wouldn’t want to be rushing in and changing the glow plugs, until the car has been inspected.

I would be doing a search for a trusted local diesel specialist, and getting the car assessed there. It might be worth asking for recommendations of where to go, at your local taxi ranks. These chaps need their cars sorted efficiently and quickly to maintain their income. 
Perhaps you could let us know how you get on Danny.

Kind regards,

Gareth.