This sounds like patellofemoral syndrome. Degeneration of the back of the patella (or kneecap). Difficulty w/ stairs, pain after sitting, and trouble getting up are hallmark signs.
Unfortunately, the cycle goes something like this
Pain--->quad atrophy--->more pain
Would advise physio to help build the quad. Terminal knee extensions are the safest and most effective way to build quad in presence of patellofemoral pain. Should also utilize an elliptical as bike riding is a no-no for PFPS (bike riding better for tibial-femoral pain which is by far the much more common osteoarthritis). Applying BFR bands before using an elliptical increases metabolic stress and type 2 muscle activation- which should help accelerate quad recovery.
More Muscle--->absorbs joint impact--->less pain (and better function)
Build the quad!!!