I've been Googling Epping Forest and Trent Country Park and they both look like perfect spots for getting really muddy! Being rather far north from me in Balham, however, I think these ones will be good Sunday long runs as it will be a good hour or more to travel there. So Sezz and Gavo, thanks for the info, you've sorted out my weekends for me!
I'll also be checking out Hampstead because, as Rowan says, it doesn't look like it's that small from Google satellite photos! I also want to check out the bathing and swimming pools/lido there too!
Richmond is going to be closer to me, only about 20 mins by overland train and bus from me in Balham. So there's another weekend day that's sorted. Tooting Bec common, wandsworth common and Brockwell Park I think are going to be my main evening runs.
Anyone know what Battersea park is like? It looks quite sculpted and well looked after in photos? I've been running in Liverpool for a while now and I absolutely love running down the River so I think that park has to be on the list too!
Thanks again everyone!