20 down is "Labour Day delivery (3)". I have S?N. The obvious answer is "SUN" i.e. The Sun newspaper which I used to associate most with supporting Labour, but surely that cannot be said of the recent election. "SUN" much surely be correct, but I would welcome confirmation/support!
I hadn't thought of "Son" but I think it has to be the best bet. I would hope that if they were looking for the name of the newspaper they would be looking for "THE Sun".
Many thanks to you both.