You are not paying attention. The only thing I've said you should change from the original field code is Data. Change Data to whatever your mergefield name is, not its cell address or something else, the name (i.e. what you see when you insert the field via Insert|Mergefield).

One change that should be made, is to add another REF to the field code, so as to ensure the '£' is applied correctly. Instead of:
{QUOTE{SET Val {MERGEFIELD Data}}{IF{Val}= {=Val} {=Val \# £,0.00} {REF Val}}}
use:
{QUOTE{SET Val {MERGEFIELD Data}}{IF{REF Val}= {=Val} {=Val \# £,0.00} {REF Val}}}