Watchkeepers observed RIB stopped in channel. Pan Pan broadcast by Solent CG re-RIB broken down. NCI confirmed to CG that they had RIB visual. Later call to CG advising RIB still adrift. Informed vessel has no VHF radio, but occupant has adequate clothing, lifejacket and flares. CG in contact via mobile radio. Exmouth ILB launched and towed RIB to Marina for CRT check. Pan Pan cancelled by Solent CG.