If it's anything like my Toledo, which looks very similar to the 90 from the outside, the grill door has no lock, but it's held shut by an internal closing spring beside the left-hand hinge. Access appears tricky if that door is stuck shut, and I can't think of any simple solution. Let's hope that someone else has some ideas.
I can only think (always assuming there's not a lock on yours) that the spring may have broken or become dislocated. It might be worth squirting some WD40 down the left-hand crack and very gently attempting to lever it open using something non-metallic - a plastic or wood kitchen stirring spoon for instance - but being very gentle and careful not to chip the enamel, cause other damage or get a bit of the tool broken and stuck. Once it's open, the problem is straightforward as all the access screws are there.
Have you tried calling Rangemaster for advice, or even your local dealer if they offer repair facilities?