Well the problem seems to have been fixed for the price of a wooden skewer, which I already had. While trying to find a solution which meant not having to spend another hundred pounds on another camera which may, or may not, work, I had another look at the CHDK Forum. I was looking for a possible intervalometer script which can be programmed not to run at night, to enable the S5IS to be used instead of the faulty S3IS. I found one but as usual with these things the link seemed to be broken. But while noodling about i found this:
http://chdk.setepontos.com/index.php/topic,2075.0.html
Which provides a genius use of a skewer to reduce battery consumption by tricking the camera into thinking it is connected to a tv. I must do this as well, but in the mean time it gave me the inspriation to come up with a simple, lo tech fix - and it seems to have worked. Hooray! Plus it means i have a larger capacity card, so the HDR option is also back on! I am testing it again to make sure, but it feels pretty solid and I have a lot of confidence, I will need to address some other problems caused by the new camera (position and access). But now at last i can get going...