Strange activity which I can't explain

On the 20th of April this year I’ve got a huge dip (£170K) in my holdings which only lasted one day. I’m pleased to realise that there wasn’t a huge market dip on that day, but it must mean that there is something to do with my activity on that day that caused the anomaly.

Here are the transactions from the days surrounding the 20th of April. Can anyone see something that I can’t?

Please read the solution to this post: Portfolio Value graph incorrect