Hi Roof, just for clarification; I only do the crosswords online as I don't live in the UK. When doing this one online if you select 5 across (and by selecting it, it 'highlights' in yellow on screen) 20 across is also 'highlighted' implying the clue is solved as one answer for 5 and 20 across.
Because the clue for 20A starts with the words "See 5 across..." I assume the editor has seen that as an instruction to link the two answers where in fact there are two separate clues and two separate answers. Hope this clears things up and sorry for boring you all.