When I pess any singlr Key to programme patterns in step and/or tap mode I always get two buttons lit up, the one i press + the next but one. Making it impossible programme properly. Is it a fault or is there somthing else I can do? Help much appreciated.

Is this before or while your playing the pattern? If its before, then you are selecting patterns that are chained to that next pattern (so one plays, then the other)... if you clear a pattern (with the clear button), they wont be chained together anymore.

It's before I play the pattern.
I clear the pattern with the clear button,
select an instrument, then try to make, for example, a bass drum on every four note, but instead of getting a four four beat I have the drum on 8 notes, on off, on off across the keys. And thats the same for each instrument?

so you're trying to program patterns without it actually running? I think you have to have it running to program in beats... im pretty sure. i dont have a 909 here to check and its been a while since Ive played with one.

No, it's while I play the pattern, the 909 is running in either mode, but any attempt to programme a pattern is always resulting in double the no of notes I want, I press one Key and always get 2 registered and lit up, always the next but one. Absolutely infuriating, and in fact impossible to make sense of, unless there is a circuit error?

I imagine that would be quite frustrating. It's possible there are circuitry problems, or maybe just sticky buttons.