Stage 16 : Brecon to Breacon Beacons Reservoir, 8.8 miles
The official Stage Route Map can be viewed here (as a pdf file).
The stage route can also be viewed here on Gmaps pedometer which includes elevation details, although it must be stressed that the distances and altitude calculated by Gmaps are approximate and not race measured.
Race marshalling and other instructions for this and indeed all 20 stages of the relay can be seen here in the 'Castles Reference Pack'. This quite a large file though (2.9 Mb !).
Start: Wellington Hotel, St Mary Street (Grid Ref: SO045284).
Start time: 11:55 am
Finish: Grass verge just before car park entrance (Grid Ref: SN983203)