Any recommendations as to the best products on the market to do this – Grangers, Nik-Wax – any other testimonials?

Either will do. Just follow the instructions – to the letter

Nikwax wash in stuff has always worked well for me, never tried Grangers.

I had a works visit to Gore recently.

Liquid non bio detergent and spray on DWR was the clear recommendation.

During your works visit, did they explain why it leaks in the first place? I mean, it’s a membrane laminated between ripstop nylon. Replacing the DWR does nothing for the membrane except create a path for moisture to escape. The membrane is waterproof regardless. If the jacket leaks the membrane is damaged.

I had a meeting last year with a Gore technical team, different subject, but I did discuss this with them over dinner – the only answer they could give me is that probably I purchased the wrong jacket and that the rucksack straps had damaged the membrane.