The go-to diagnostic tool for the DIY mechanic with Audis (and other VAG vehicles) is VCDS by Ross-Tech. The official retailers in the UK are listed on the following page (scroll to the bottom):

https://www.ross-tech.com/distributors.php

The majority of the ones available online elsewhere are clones, and may or may not work, but if you're planning on keeping the car long term, it's a wise investment, especially with dealer prices.

As the saying goes, 'other brands are available', such as OBDeleven and Carista, but I have no experience of them, so can't comment on suitability.

If you get a decent scanner, it's worth uploading a full scan here, as some people may have already experienced the same issues.

As these cars are overly complex, I'd suggest you refrain from throwing parts at it for now until you get a decent scan tool, as the root cause could be something completely different, and you would just be wasting money.