Trading losses can be relieved by S45 against profits of the same trade in "subsequent accounting periods" i.e. indefinitely - periods, plural.
But relief against total profits is stated by S45A to be relievable in "the later period" and by S45C in "the further period," in both cases the word period being singular. I can only read this as meaning the relief is only available for the two years following the year of the trading loss. But as I've not seen anyone, anywhere make reference to this significant restriction I have to assume I've understood it wrong. Any thoughts (or even better, authorities)?