Don't even pretend to know anything about gardening but

[earlgrey]

we've got this huge, massive even, lavender bush - but with about 5 spikes of lavender growing out of it. Every day I have to walk past this equally huge bush that has tufts of lavender coming out of every leavy thingy.

What am I doing wrong? Mum says I ought to prune it, but since she's a Londoner and never had a garden I'm inclined to accept her advice without necessarily knowing it's true. And how and when are you supposed to do it?

Yes prune it when it has finished flowering but only lightly - lavendar will not grow back from old wood.
BUT , its a quite a short lived bush so it could be too old in which case you need to replace it - but try a light pruning first.

If it's massive I'm wondering the same - if it's old and mostly dead.
I'd probably just dig it up and replace it - I planted a little one a couple of years ago and it's doing really well, and I have zero gardening skills.