Numbers formula help please

Hi I'm trying to complete a spreadsheet in numbers and I’m struggling. My husband use to help me when the formulas played up but he died recently so I really need some help because I can’t work out where it’s gone wrong!

It’s a speed sheet for very simple accounts, first boxes are date supplier details then gross vat and net. Then it goes into sub heading boxes and then the last box is a ‘check’ box - Other- P so if I’ve not imputed the amount into the sub headed box correctly then the amount goes into the other P box and I can check it and realise my mistake.

what’s happening is the first sub headed box G which is sundries if I put a number in that the other P box is doubling the number and putting the amount in the box

so net £10 put it into , sundries G = £10 in the P box it will say £20 it should be £0.00 because the figures are right.

Im really struggling to see any difference in the formula and wondered if anyone can help me, before I spend another 2 hours looking at it and crying. First picture is the formula which is working the second isn’t working!

thanks