Following on from the previous post, a friend of mine has recommended that about a pint of fluid (568ml) every hour should keep you hydrated. That sounds like a lot - one bottle + a little bit more of Lucozade Sport every hour - but I suppose it makes sense especially in the hot weather.
The latest forecast for Saturday is dry with variable cloud, a moderate breeze and moderate temperatures 16-18 celcius. There is apparently a small chance of showers in the evening/overnight.
This sounds to me like ideal conditions. Blazing hot sunshine can be very uncomfortable to walk in, especially where there is no cloud cover or breeze to give you some respite. It's nice if there's a bit of wind and I'd certainly have that than hot sunshine all day. I'm a bit mixed about rain - I don't mind it if it's light but anything approaching last year's monsoon will not be good news!