I have got to say, my feelings are rather at odds with this forecast, mostly for the earlier part of the summer. I can't claim the level of experience and expertese as Stewart I must also add.
However I am thinking along the lines of this summer being something of a slow developer, with a rather cool and unsettled June, but warmth developing into mid/late July. As for August, too far out to say. I don't think we can expect a June CET any more than 0.4 degrees above average (and that will mostly be minimum temperatures) and it could also bring the SE the much needed rain. In July I'm thinking of more consistent mild temperatures rather than any "heatwave", but lasting longer than any of the 4 previous summers have produced, and dryer than average.