This is both annoying and alarming, 'eh?

I removed all the conditional formatting from the sheet.

Would you try that?
For me, the results were that only G10, G16, and G21, had extra wrong bits added.

Doesn't your original have more cells that than with wrong values? I think G30 was wrong when I first looked at the sheet with conditional formatting.

Makes no sense to me, I've seen plenty of simple formulas work just fine, although I'm using USD and not GBP.

What happens if you re-do the sheet using USD? With or without conditional formatting?