I have attended these walks before and they are a lot of fun. The cost is £1 per dog (really cheap!). This walk is on-lead but there will be several stops on the walk where you can let the dogs off-lead if you want. Between 1.1/2 - 2hr walk.
I have never been to this lake before and will go dependant on the weather.
For more information have a look at the web-site http://www.theparkstrust.com/whats-on/event-details/784