Either:
J9:
=SUM(INDEX('london plan'!$C$4:$C$31,MATCH($C9,'london plan'!$A$4:$A$31,0)):INDEX('london plan'!$C$4:$N$31,MATCH($C9,'london plan'!$A$4:$A$31,0),MONTH(1&$B$1)))
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or
J9:
=SUM(OFFSET('london plan'!$C$4,MATCH($C9,'london plan'!$A$4:$A$31,0)-1,0,1,MONTH(1&$B$1)))
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the latter is volatile unlike the former (semi volatile)
Regards your PLAN month column:
F9:
=INDEX('london plan'!$C$4:$O$31,MATCH(C9,'london plan'!$A$4:$A$31,0),MONTH(1&B$1))
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or
=SUMIF('london plan'!$A$4:$A$31,$C9,INDEX('london plan'!$C$4:$O$31,0,MONTH(1&B$1)))
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I prefer the latter.
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