Hello from Steved
The formula below works,

=HOUR(IF($T$40.5,$T$4-0.5,IF($T$41/24,$T$4+0.5,$T$4)))
&TEXT(MINUTE($T$4),"00"))

but when I add IF(ISNUMBER($C$4),the cells go blank which
is what I require, however when I put in a value, it stays
blank. Is their another solution to blank the cell please
as there are over 900 cells which I need to have blank if
there is no value to go in. (It is showing 1200, until i
enter a value.)

=IF(ISNUMBER($C$4),HOUR(IF($T$40.5,$T$4-0.5,IF
($T$41/24,$T$4+0.5,$T$4)))&TEXT(MINUTE($T$4),"00"))
Thankyou.