Hi, I am not an expert. But I have owned the same car and that had auto box problems.

You wrote: ‘Wonder if anyone can help with my Grand Voyager problem - the car is in drive, but when the accelerator is pressed the engine revs but the car doesn't move. Then suddenly it may lurch forward but then stop again and the engine revs again. It reminds me of a slipping clutch on a manual.’

This I think is what they call a “drive plate” problem. However that said, if you do not hear a noise then it is unlikely that the problem you are having is the drive plate. And when you write the following:

‘Checked the transmission fluid level - very low. Topped it up - the noise that used to be there on engaging drive had stopped, but still revs high, moved a touch (no more than before) and suddenly the steering went stiff, like the power steering has gone.’

Here above, you introduced a “noise” issue that you had not mentioned in the beginning. Therefore still depending on the type of noise the car could have a drive plate problem. If not and even if there could be another issue that being the “tensioner” that is the “belt drive” I write this because these make a noise, when they need changing, and I am not solely referring to a slipping belt noise but moreover to the noise of a barring on the tensioner when on its way out. The tensioner problem is for me your starting point, since if this is the problem and not the drive plate then you are laughing because not that expensive to repair that is replace. I know this because I had mine changed when I owned same car. Low cost parts and labour for this kind of job. If the drive plate too then whole job including parts I paid £563.04. I hope for you that it is something like the tensioner / belt drive because you can get this sorted for around £100 all in. (if you go to a Chrysler mechanic. (Not main dealer) Because they know what they are doing and cheaper than main dealer. You wrote that power steering got hard. Well that tells me that the problem is most likely the tensioner as I wrote rather than drive plate unless your car also has the drive plate noise which is a rattle sound. It gets louder as it gets worse. And if the drive plate it interferes with the timing of the engine causing the car to move faster than speed selected and even causing car not to move at all. Moreover can play games with auto gear box fluid levels. Also make sure you put the right kind of oil in the box. If you do not have Manuel then ask a shop that sells oil or the main dealer. It is important that you use the correct oil. Also check for any leaks, I write this because the sumps sit low on this car and get it by curbs etc, so could be a leak in the sump. Leave some cardboard under the sump when you park up and in the morning or next day or after a few hours check if any leaks.

There are other things that could be the cause, but I only write what I know from experience. Hope all above helps in any way. Good luck.