The principle problem to overcome when making strawberry jam is their lack of pectin, which, according to Darina Allen's Ballymaloe Cookery Course, is a "mucilaginous substance" (tasty) that rather handily, acts a setting agent. The more acidic the fruit, the more pectin it is likely to contain – so the intense sweetness of the fully ripe strawberry counts against it here.
Fear not: there are three ways to up the pectin quota: firstly, with the juice of a more acidic fruit, such as lemon or currants, secondly by using jam sugar (which includes pectin), and thirdly by adding some in liquid form to ordinary sugar (some sticklers regard the use of artificial pectin as cheating, but with strawberries, I suspect they're making a rod for their own backs; it just provides extra reassurance that you're going to be able to spoon, rather than pour, your breakfast condiment).
Jamie Oliver, however, has no such qualms. He uses 500g jam sugar to each kilo of fruit in his strawberry jam – but without a spritz of lemon juice, the results are overpoweringly sweet, even by jam standards, and the set is quite loose – it slides all over my scone. His nan, seen tucking in at the end of the video, may approve, but it reminds me slightly of the sickly pink syrup that adorned the 99s of childhood: not the kind of nostalgia I'm after.
Nick Sandler and Johnny Acton go down the opposite route in Preserved, using ordinary white sugar and lemon juice (2 lemons to 3kg of fruit). The set is much better, and the jam has a richer flavour – as Pam Corbin, aka Pam the Jam points out in the River Cottage Preserves handbook, the acidity helps the jam "come alive".