tt2 - thanks very much, this worked!! In summary therefore I have done the following (after installing iOS12):
1. Updated from Yosemite to High Sierra (to do so I was forwarded a link by an Apple support team member, I couldn't find it just by going to Apple);
2. Removed all USB cables and tried a new lightning cable (don't know now whether this was required);
3. Re-installed iTunes from tt2's link;
4. Re-connected my iPhone (a further download occurred) and then the sync started just as it used to.
However, Photos did not open and/or update - might I need to re-install Photos as well?