Clause  20.11 Says " If a player returns a Qualifying Score or Scores below his Playing Handicap at his Home Club (see Notes)
or at an 'Away Club' and is unable to:

(a) report an away score(s) to his Home Club or
(b) ascertain whether or not his Playing Handicap has been reduced as a result of the score(s)

He shall before playing in another competition at his Home Club or Away Club, for that competition make such reduction to his
Playing Handicap as shall be appropriate under the UHS by applying the Competition Scratch Score if known,
otherwise the Standard Scratch Score, to calculate his Nett Differential and handicap reduction.

Should a player not know his Exact Handicap, such reduction should be made from his Playing Handicap less 0.5,
(e.g. if Playing Handicap is 16 then the reduction should be from 15.5)."

Notes:
Handicap reduction only shall be applied under this sub-clause.
Handicap increases may only be made by a  player's Home Club after scores have been duly reported.

Supplementary Score (Extra Day) Cards are considered to be a Qualifying Score.
(See Maintaining Your Handicap)

For Example:
If a player with an Exact Handicap of 11.3 returns a score with a Nett Differential of -7 (being number of shots below Standard Scratch) his Exact Handicap is reduced by 7 x 0.2 = 1.4. i.e. to an Exact Handicap of 9.9 and his Playing Handicap is 10. See table below, this Playing Handicap should be used until advised otherwise by Home Club.