Canadapilot, you can cross the pond below FL 285 or above FL 420 without specific navigation equipment.
But if you want to cross it within the MNPS airspace (FL285/420 excluded) you have to comply with the Minumum Navigation Performance Specification. In first you find the minimun requirement in the MEL, on ground, other information and/or restrictions affecting further failure(s) are depicted in the MNPS manual and resumend in the AT(H/L) 1-2 Jeppesen chart.
Specifically to the Bus You can find navigation requirements for RVSM airpsace in FCOM 2.4.45 and remote areas flight in FCOM 2.4.46.
NAT details here:
General site
North Atlantic MNPS Airspace Operations Manual
Enjoy, HVIII