Underground gas cistern and plants

[woolybanana]

In the front garden there is a rather ugly rectangle of ground above the underground gas tank which looks to be topped with some sort of light coloured gravel. Buddlea is self seeding there which got me to wondering whether it is ok to plant stuff over these tanks or not, excluding the rectangular inspection chamber, of course.

If not, I would like to put plants in pots there if this is ok.

Any thoughts welcome please.

Thank you in advance.