I've recently bought a 2006 A4. I have noticed that there is a groove wearing on the inside if both front tyres. 

The car has 18 inch wheels fitted which I suspect are too big. Has anyone fitted 18s and had the same problem 

I'd recommend getting the tracking checked, as well as having the suspension checked. Predominantly the suspension arms as these could be affecting the tyre wear. 

Welcome Andrew and thanks for joining. 
By your description of ‘a groove wearing ..’ would suggest some component is touching the tyres at the site of the groove - either suspension related, or a steering/ suspension component, rather than an alignment issue. 
You should be able to determine where by turning the wheels on to full lock and looking under the car. 
However, as Steve says, it would be well worth you getting the car assessed at a trusted local tyre retailer. 
Regards,

Gareth. 

Hello Andrew,

You seemed to have joined and posted at the same time, and not returned since to read and acknowledge the advice you asked for.

Any particular reason?