Obviously the longer daylight hours in June help production, but low temperatures will increase production too.
<nerdy bit>
PV panels typically have a negative temp coefficient of around -0.25% to -0.3% Volts per degree C, with their rated output based on cell temperature of 25C. So in high summer cell temps (in UK) are likely to be 40 to 50C, whereas this time of the year they will be much lower. So, for example, if the panels are 25 degrees cooler you'd get about 7.5% greater power. Which is why peak production time of the year is commonly end April / early May.
</nerdy bit>
Maybe it is very cold in Dundee right now