My husband and I have recently used a company that literally transformed our carpet. We didn't even realise it was that dirty until it was cleaned. The original colour was fully restored, it really looks like a brand new carpet. The main reason we wanted help with carpet cleaning was due to a rose wine stain on the surface. It was completely removed.
They use a specialised tool that extracts the grime with the use of steam, I believe. If you're still looking for some help you can contact them on: 017 5338 6166 - the company name is magic pro cleaning. They've given me a business card. This is their website too: https://magicprocleaning.co.uk/