Beddau took the lead with a try by left wing Richard Bull that fullback Steve Robinson converted, but centre Kevin Russell's try, converted by fullback Geraint Walsh, levelled it.
Beddau stormed on and flanker Lloyd Rose and centre Huw Williams notched tries, both converted by Robinson, before the interval and fly half James Balfrey then added a penalty.
Llantwit responded with a drive that saw lock Tony Davies score and Walsh convert, though Bull again crossed for Balfrey to convert and make it 31-14. Llantwit still came back with Walsh scoring and converting at the finish.
The final will now be played out at Virginia Park, Caerphilly.