1.8% error in ETF Securities' physical gold ETFs PHAU and PHGP?

There was a near 2% drop in the price of ETFS Physical Gold (PHAU) on Tuesday that didn't appear to correspond with the spot price of gold. The drop happened after a large trade of the ETF for the day (an automatic sell order  of 11,000 shares worth around £1.8 million).

According to trading data from London Stock Exchange, a minute later a much smaller investor sold 361 shares for £58,600 at what looks like 1.8% lower than the spot price resulting in the investor losing more £1,000 in the sale. (click on image below to enlarge, the bottom two trades are the ones I'm looking at)

At least that's what it looks like to me as I can't see any corresponding falls in the gold spot price provided by BullionVault (immediately below) - or in the PHGP price in the chart  below  - but I'll check with ETF Securities.  

Something similar happened to the sterling denominated PHGP too:

The ETF Securities' sterling denominated physical gold ETF (PHGP) dropped from 10,394p to 10,215p, a fall of 1.7% at about 3pm.

Data  from the London Stock Exchange shows that an "ordinary trade" went through at that price at that time.  I don't know if that price was available to anyone else. (click on image below to enlarge)