Looking at megatrain.com I can leave Portsmouth Harbour early in the morning but the times for decent departure are as follows:
Depart
Return:
So £16.50 all in, the departure is rather early, means I'll be wandering around for a couple of hours!
Alternatively I could use my train travel discount from work and get the same price via SWT and jump on a train for wherever in London I guess and not have to pick specific times. I'll find out on Tuesday!
Where are most people arriving at train station wise? Waterloo?