Hmm, solved my problem, figured the tank was driving a constant circular route round the town so just hid down an alley and waited for it to come past
And I thought I was good at these games

Hmm
Mopped up a few soldiers, and that was it - end of game.
Now to play it online a little more and also go back to Serious Sam 2.
I'll also give it a while and play through on a harder level.
One of the end credits: 'Thanks to all the men and women from around the world who gave their lives to give us our freedoms, we will never forget you'.
Final credit: 'No cows were harmed in the making of this game'