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Thanks, this sounds like a much more professional method than I had in mind and I have some spare plain skirting lying around.
Thanks @hugo7 and @Howard also. The floor slopes down to the right (or the gap slopes up?) so the gap gets smaller to the left. Used to be filled by carpet. Filling feels like less work for a satisfactory finish than replacing the skirting but maybe I'll learn that's not the case...
Geoffnotsle-
At our gaff whoever had the additional wood flooring put down over the boards (it's that engineered stuff that's now £££ and much shitter than it was pre-pandemic) just butted it up to the original skirting then tacked on another moulding to the skirting to create a lip that overlaps the new floor. It works if you don't really know the horror it conceals. In your case it might be a goer - it's dead cheap and simple (you still need to mitre corners tho) and removable.
Howard
If it was me I would mitre an end of a piece of plain skirting 1/3 the height, place it spaced a few mm off the floor and mark along it’s length and then multi tool cut the lower piece out and then fit the new lower piece, test fit so it’s as neat as possible probably using some packers to get it flush then fill, sand and paint.
You could remove the piece first to check there’s something decent to fix to and the width of new piece to buy.
A tradesman would probably have a quicker easier way.