Hello folks
I have done more reading and am replying to myself here in the hope that others seeing this thread find it useful.
The reason I'm getting all of these side shoots, despite pinching out, seems to be because San Marzano are so vigorous.
The seeds I have planted are for indeterminate culture. So one should nip off the side shoots.
There are other varieties of 'San Marzano' which have been crossed with other varieties to produce bush (determinate) varieties. Yes - still called San Marzano apparently.
I have googled images of San Marzano plants being grown commercially in Italy and they all show the tomatoes grown as indeterminate plans. Interestingly the lower leaves, up to about half of the plant height, have been removed to allow for better ripening toward what seems to be late season.
Some gardening sites recommend allowing two or three side shoots to develop, from low down, into stems to grow to full height. Full height is where you might stop them by nipping off the top growing point.
I am going to go along with Jezza's recommendation and allow some side shoot development. Simply because they are prolific with flowers.
Any comments on this are most welcome.