Is there a definitive postcode (outcode) map online?

All the maps I have show Mayow Rd as being half in SE23, half in SE26,
whereas numerous signs on the road itself say that it is all in SE23...
however, signs on Lockwood Close show that as SE26, and I can't imagine the
post office putting a dead-end road off an SE23 road in SE26. However, 2
sign errors, one rather bizarre, within a half-kilometre of Mayow Road cause
me to have no confidence in Lewisham Council's signage department.