to think Delia and my mum were lying.

[trio38]

According to Delia AND my mum, cooking the fruit for strawberry jam will take approximately 15 mins, so AIBU to wonder why the fuck I'm standing here 40 minutes later waiting for the sodding strawberries to show signs of setting?

of set blackberry jam, we have pots of liquid blackberry jelly. Normally it's great, but the last batch just would not behave.

Plums are good.

And my DM always used certo when doing strawberry jam, which she made in heroic amounts. I know we were the slave labour used to pick the strawberries

If you add apple it helps it set. I can't think of any jam that would take 15 mins from start to setting point - I'm sorry OP but you have been sadly misled by Delia (OK she's stranger and shouts at football matches) but also your Mum!!

If you want to make jam, in my experience you need to set a side a whole day in the kitchen and prepare to have every utensil, all your clothes and the kitchen floor covered in jam. After the repeated testing process you will have about two tablespoons of jam left to put in a jar.
The feeling of slight stickness will remain throughout the whole house for approximately a month afterwards. You may want to embrace that sensation rather than fight it.

Boiling sugar (even with other ingredients) acts in the same way that it does if you make toffee ... so the longer you boil the jam, the harder and harder the resulting 'toffee' will be!
"Setting" is totally different. Its about levels of pectin (the setting agent in the fruit) not about length of boil. Boiling is only about releasing the pectin. Unlike dropping toffee into cold water and needing soft ball for fudge or hard ball for cracknell, literally all you need for jam is a "wrinkle" on the surface.
Strawberry is notoriously difficult.
If you have had issues, try adding lemon (high in pectin) next time - or buy a bottle of Certo -liquid pectin- as mentioned previously.