Most "fabric roof cleaners" are simply diluted APC (as are most specific cleaning detailing products on the market) What would make it an effective fabric roof cleaning product is if it contained and enzyme eater which would help to kill and remove any moulds or live bacterias.

As previously mentioned, you shouldn't need this level of cleaning power at this stage but saying that, fabric guard products need a meticulously clean substrate to work properly so make sure it gets a very good clean with an APC then FULLY dried, this is also of the utmost importance as any moisture will halt the curing process.

I am also a fan of the Renovo products. Application with a brush is far superior to simply spraying as it penetrates the fibres for a better coverage. Carpro Fabric has the best longevity in my experience (around 18 months if applied correctly) but it is pretty unforgiving, as are most. Unless you have the suitable environment and time, I would recommend going with Renovo, which is a little more forgiving and is £30 cheaper than the likes of CarPro Fabric.