Is a lack of fertiliser VERY detrimental to the final yield?

Yes, but people often do more harm by over-fertilising.

Start with good compost and don't dilute it, repot several times, finishing in pots of a decent size and you're 80 percent there.

thanks for the quick reply. how many times is it good to re-pot? just had a crop that was very below expectations.

My current strategy is to sow directly in 7cm pots, then repot into 13cm pots at 14 days from seed where they stay for another two weeks.

Tonight I will be repotting into 18cm (6.5 litre) pots for the final week (or two) of vegging.

In my opinion it's a balance between training the roots to use the compost effectively, and not making the plants "bolt".

not as much as over ferting is
and Welcome to UK420 head man

redgrotto