All of a sudden my speedo has stopped working again. It stopped a couple of months ago and i got the speedo sensor replaced, now it has gone again and i have no idea why? All i can think may contribute to it was that it stopped the day we had the downpour - can water get in and short a fuse? Is there a speedo fuse?
Also, my car now won't go over 3000rpm; you can put the pedal to the floor but it won't go any higher. No engine warning lights or error codes.
Any idea what could be going on?

It sounds suspiciously like the brain's had a lobotomy. I would suspect it's your ecu or something close in the circuit that's gone wrong.

Well i drove it this evening - and speedo is working again and i can go over 3k rpm. Go figure.
Although a little worried, as you say, this may indicate bigger problems going on.

Well, the speedo has stopped working again. The only thing i can narrow it down to is that it seems to happen when it really pours heavy with rain. Normally a few days later the speedo jumps into life, but this doesn't seem to be happening. Tried disconnecting the battery, no difference. No error codes. Floor in passenger foot well is also damp. Since i've already replaced it before i'm not sure i want to again, but is there any other reason it would stop working?

Check the sensor and the wiring could be shorting out it getting soaked. Find out where the waters getting in first see if that cures it.

Any suggestions on how to find out where the water is getting in? Are there any usual culprits? I thought i'd sidestepped this problem by getting a vehicle without a sunroof...