On excitedly informing Mrs RAPC of the chance for a weekend break just now, she was good enough to point out the error of my ways.
I had forgotten 2 small things in my excitement to put me down as a possible:
1) It is my wife's birthday that weekend
2) I've already booked for us to stay in London that weekend
Which when added together makes me an absolute idiot.
Sorry I can't make it, which I'm gutted about. Have a great time, hope you get the picnic sorted and enjoy yourselves!