I have actually read an interpretation (from the CAA) somewhere that if there's any doubt about the conditions of the field, that it is perfectly legal (wrt. rule 5) to inspect the field by overflying it on subsequently lower levels, even below 500', as long as you intend to land there after your inspection finds out the field is OK.
So don't do a mock approach to land and throw it away at 50', overflying the field on the centerline, just to have a good story in case you get busted for rule 5, but do a proper (precautionary landing-type) approach right of centerline (assuming you're in the LHS), then remain at 50' to inspect the field. If the inspection is allright, land on the next pass.
On an unknown, unmanned, unlicensed, grass field, with no recent pilot reports, I would say that this is normal aviation practice.