A few days ago my N900 was dropped into a cup of water/coffee, I l followed the standard method of removing battery, dismantling the device and leaving to dry in a tub of rice for 2 days.

To cut it short it worked fine after I put it back together but after about 5 minutes the keyboard would act up and random buttons would stick and be constantly pressed crashing my device

Again I disassembled it and cleaned with alcohol which seemed to have sorted the problem out and I was very happy again

But then after it started having another problem
the Fn keys seemed to be stuck on so instead of seeing 'q', '1' would appear and it was the same for most buttons 'v' = '/' etc.

I flashed the firmware hoping it would sort it but it hasn't and I haven't a clue what to do now!?!?

It seems no-one else has had this problem so any help would be so grateful, I thank you in advance.

I would try to go back to the alcohol solution and see if you can get Fn keys to be unstuck.

Apart from there there isn't much rescue for your N900 as it may over time face issues with metal oxidation and possible short circuiting.

Try leaving your phone in a bowl of dry rice overnight. This method has actually worked for a lot of people.