Your account is in another currency than GBP. Sunday evening at 10pm GMT the forex markets open (It is Monday morning in Sydney at that point). Forex markets are open 24/5 between Sunday 10pm and Friday 10pm.
If you have, just like me, an account with EUR. And you buy an ETF/Stock which has their price in GBP. The forex rate is influencing your profit/loss, because your account is in EUR. If you buy only instruments with EUR prices you don’t have any forex influence.
@Enlil the question was why were my returns fluctuating at 10pm on a Sunday. And the answer seems to be, because Forex market opens at that hour, due to Austalia timezone!
Forex was the “why” part, I was trying to answer the “how” part.
But my answer was inaccurate. What you see has nothing to do with the fund currency but indeed with the currency you buy in as stated by @Chantal. Total fail. My bad. Post deleted