I guess you mean 550mm? 5.5m is quite a dig!
In any case there is no specific requirement on depth for private land, only that it has to be adequately safe from any likely activities. Even for the electricity supply utilities like SSE putting in large non-RCD protected cables the depth depends on the land use so 45cm for footpath, 60cm for public road (carriageway), and 1m for arable land (i.e. ploughed field).
Given it is in concrete and so not going to be accidentally hit by a gardening spade, etc, I would say it is suitably protected.
Was anything specified in any contract or quote?