As above…..I would park at Brandon Country Park. It’s only £2 and doesn’t get locked.
To get from there to the trails, head back out to the main road & go straight over into the forest. After <50 yards, there is a fairly discreet trail turning to the right. Take it. If you go 100yds you’ve gone too far.
It’s a nice swoopy trail that drops you down to the bottom of the beast, where eb2429 mentions. It splits a few times on the way, but as long as you keep the general direction constant the trails all rejoin at the bottom of the Beast so it doesn’t matter what path you take….
Turn left when you pop out & about 30-50m up, there’s a trail marker taking you back into the forest; this is the tail end of the trail which takes you back to the start…..
Or – for a more direct route to the start of the waymarked routes, turn left when you pop out and head up the fire road (ignore the waymarker). After about 3-400m it bears to the right, but take the trail that goes straight on. (a bit more grassy with some sandy ridges).
Follow this to a T-junction onto a sandy path & turn right. Follow this for 1/4 mile or so and you’ll go past a field on the right. Just after this field, hang a left onto a wide gravel trail. This takes you to the start of both waymarked trails.
I would choose the ‘straight on’ trail when it splits (the other trail goes right).
Both trails are around 9 miles & you can easily get a lap of both done before it gets dark & get back to the car.
Both trails spit you out onto the sandy path near their ends, so heading back to the car is easy. Just keep an eye out for that, then turn left – finding the left turn for the ‘t-junction’ I mention above is a bit trickier, but if you go steady you’ll see it.
Then just keep going until you get to the car barrier at the bottom of the beast & turn right back up the trail you started on (this is well hidden by bracken and is closer to the barrier than you’ll remember it being…..)