In theory, but a fault that exhibits as a narrow section of noise affecting just some of the upload bins can cause this

Understood , so the OP could have a bad or noisy line,?

That or some noise source locally that is at very specific frequencies

UPDATE: BT went to the cabinet and fixed the problem. Took them less than 10 minutes. Upload is back to normal. I never would have expected it to be cabinet side but I am happy to be surprised!

Does anyone have any idea what they may have actually done in the box to fix it, I'd be very interested.

Tried to update thread title with resolved it's not having it.

Thanks all for the help replies and thanks to Zen for sorting it out super fast.. oh and Andrew from Openreach, my mum, my dad... happy Christmas all

Might be a lift and shift to a different port, or some connection was not fully home, and a bit of noise introduced but with a limited frequency range.

Or even simpler a reset of the line card port.

Interesting! Thanks!