Nial:
My MGB's top is pure white (thats the idea, anyway) and I resorted to using a vinyl/leather cleaner designed for seats - seemed to work pretty well (keep in mind I was trying to get small areas clean off a white surface, I don't know if it will do anything to a darker top).
I've also had some success with Blue Coral (I think) convertible-top cleaner. It goes on like any other spray-cleaner, worked pretty well on the white top and tonneau.
Lastly - I've used Mother's Convertible Top Cleaner on my MGTD replica. The top is similar to what you might find on a boat, it's a thin textured vinyl material, cloth-like underneath. Mother's CTC is a heavy solution that sprays on but suds (similar to a shampoo). After you get the top sudsed up you spray it off and dry it with a towel.
I don't like Armor-All on anything by the way. I used to use it a lot but there are dozens of better products on the market. On the TD-replica I used STP "son of a gun" (pretty sure thats what it is, it's vanilla scented and comes in a yellow bottle). I put a very light coat on the top and worked it around with a cloth. The top is 26 years old but looks new and doesn't have any oily residue.
One other comment on convertible tops - there are several different plastic polishes and cleaners out there that people use on plastic rear-windows and side-curtains. I've even used Meguire's Hi-Tech Yellow Wax on my MGB with a lot of success (always "looks" perfectly crystal clear afterwards and nothing sticks to it). Recently I started using "Helicopter Bubble Cleaner/Polish" by "Arrow", available at a lot of airport FBOs and pilot-supply houses. I never liked it on the airplane or helicopter plastic, but on the MG and Migi the plastic windows look terrific - and it's easy to use.