Cannot install HP CM1312nfi MFP software or run cleaner utility

Running Windows 8.1 home 64bit.  I've previously been able to install the software for my networked HP CM1312nfi printer and scanner and use the scanning software, but want to add OCR.  I installed ReadIris Pro for OCR (which came with the printer), but got the message there was no Twain driver, so I uninstalled the HP software to see if reinstalling might add the Twain.

Now I can't reinstall it.  I've been able to get the scanner to work using HPScan and Capture (a separate Windows app), and have installed the printer using Windows Add a Printer.  But when trying to install the HP software, which hopefully includes the Twain driver, the installer starts, asks if I want to install a USB or network printer, and then just shuts down when I select 'network'.  There are no error messages, it just disappears.  On checking Task Manager it is still running as a background process.

I've tried re-downloading the software, taking care to select Windows 8.1 for 64 bit, and restarted many times.  Tried installing in Safe mode - same issue.  I've tried running the cleanup utility (scrubber.exe) that comes with the software but this also doesn't seem to work, and doesn't even appear in Taskmanager.  I've run sfc /scannow which has fixed some files, but left some files unfixed.  Not sure if this is related or not, though everything else seems to be running ok.

Any suggestions or help much appreciated in getting the installer to run.  failing that, how to install a Twain driver?  I'd like to avoid reinstalling windows as I don't want to lose an entire weekend!   I wouldn't say I'm that PC savvy so instructions in plain English please. ;-)