we're flushing again.... after around 10 hours of extremely frustrating and exhausting work. I'll spare you all the details
my very good friend had to save me in the end and finally got it sorted out. He said I actually hadn't done too badly and mostly it was done. He had to screw the cistern to the wall properly and fix the plumbing tape right to stop the leak. He mentioned it isn't the type of job you give someone new to this sort of work and that his first attempt at installing a cistern was probably just as frustrating.
I only read your message just now Harold. I expect the screws are corrosion resistant since the hardware store plumbing expert knew what they were for and the type of cistern being installed.
I appreciate the time you all took to help sort this out. Thanks.