Surrey Fella

Hi all, I'm after some advice please as I am new to all this. I have a GRP flat roof. The ceiling below this is showing some brown staining, no visible signs of water coming through. I've had a few roofers come over to advise and quote. Three different ways of resolving this issue have been mentioned: sanding/cleaning/using matting and Ultraflex, sanding/cleaning using Desmopol and lastly cleaning and using Everbuild Liquid Roof. Whilst they all sound like they all do the same thing, what is the difference between them all and is there any one in particular that is better than the rest? Thanking you in advance.