Park on the side of Reacliffe Road, off Beat Road, just off the A523 and walk along the path towards the lake. At the far end of reservoir is the end of a miniature railway route which runs seasonally, the track is unprotected so good idea to keep dogs on lead if the train is running along this stretch.
A bit further along there is a dam across the end of the reservoir, and over the dam is a visitor centre with a cafe, which gets quite busy. There's also a car park here so the walk can equally be started from here. To continue around the lake, follow the footpath out along the road for a bit then follow the footpath signs. The route is quite obvious...  If it seems too far to go all the way round (it took us a couple of hours) it can be equally nice to go there-and-back at any point.