I need to install a lintel for a new door opening. The opening is only 800mm wide. Its a cavity wall with internal and external brick (no block-work) with 50mm cavity. The drawing says use a Keystone S/L-50 lintel. When I have had a look those lintels are suitable for external brick and internal block-work, but I have brick both sides. Should I be concerned, is a different lintel more suitable for brick internal and external?