I've just come through a horrific time after an audit from HMRC with my current business (non-aviation related). Don't mess with them! I made a bad choice of accountant and they made things messy.
HMRC can charge up to £5K every time they look at something they're not happy with. They kept asking why my profits were so low in January compared to the rest of the year. My industry shuts down in January but they wouldn't accept that as an answer and kept trying to dig deeper. I ended up with £15K of charges because of that. I hadn't actually done anything wrong other than was late with payments. It's not worth messing with them.
Give them their 20% and give it on time!!!