Thank you Nikkipollard. I am extremely grateful for your solution.
I am running TC2018 Deluxe on Windows 10, so I confined myself to 'Microsoft Print to PDF' which didn't allow me any option beyond A3.
I installed CutePDF Writer and all looks well. I have tried it, without printing of course, and the resultant PDF looks as though it will work exactly as I need.
After tomorrow night I will be required to amend a survey drawing (A1 originally) and print it to either A1 or AO size. I will post the result - success or fail??.
What a pity such a simple, yet necessary, requirement to print out large-sized plans is not readily available in the IMSI user manual!!!!!!?? In my view the manual, while it has much good information, many procedures I have needed to use are not easy to find and it needs a gross overhaul.