I would have thought that a number was also involved in 28 across to be consistent with the other entries. That would give an expression which apparently comes from the early years of football commentary. That being said if the same number is used in 27 down it is not an expression with which I am familiar. Back to the drawing-board later.
I did look for a possibility for the last cell of 28ac but only came up with a 'B' (a musical term) and no numbers at all. I've now googled it with the number in 27dn but nothing is coming up.
I think it's just a maths term working on the two adjacent numbers independently?
I think the last square of 13D should contain the standard symbol for a mathematical operation of five letters, not a number. (Well it is a number in Roman numerals but that's not the way it's being used here). The unchanged version of 13D is thus a periodical name.
That results in a formula that leads to the puzzle's number.
Matakari hasn't been on with their thread s yet but hopefully they can help tie up the loose end's. Normally I like the Spectator normal clues but all this messing about with unclued's and hidden themes is a turnoff for me
I think the Roman numeral is the best solution (I went for the BBC option because that's where the show first aired - and because I hadn't heard of the alternative, of course) a neat trap!
...and I was also tempted by the neat way the two numbers (treated separately and placed side by side) give the sum in a different way to the multiplication.
There's an ongoing debate on the other site regarding the correct mathematical approach to take - which, I think, means there's a third alternative 'solution' to consider (so I'm going back to my original, simple, solution - it's safer).