Self assessment tax advice

[Yarrgh]

I did initially, but, after speaking to HMRC, I removed it as they have that income in the PAYE section.
The tax bill then went down to £150 ish which seems too low.
I was expecting to pay about £500, but, both figures I got don't seem to be correct.

To be fair, that sounds more likely - the £150. It does tend to go:- Low low low, hit a certain pay threshold and BLAM! where's that tax bill come from?!?