I am trying to create a holiday planner for the GP practice where I
work, and have inserted a number for available appointments a GP
has in cell B1, but when they go on holiday I insert a H in cell A1 and
it turns yellow, and in cell B1 I want it to change to override the
number of appointments by placing a 0 and also turning it yellow, I can
get the colour to change but not the number, I have used formula is:
=IF(A1="H",0) but it is not working, it works as a basic if function
but not in conditional formatting, please help.

What I'd suggest is to do two things to get the result you want.

First, use conditional formatting as you already have done.

Second, use a basic "if" function in B1; for example,
=IF(A1="H",0,5)