May be time to put some new plants in, mine seem to go like that in late summer & I usually just pull them out & start again.
I guess it depends whether you think they might be diseased or just old. A lot of plants become bitter when they are going to seed (like lettuce). It certainly doesn't sound like they'd be deficient in anything!

Thanks Sarah May ~ they're actually not all that old (planted late summer). Maybe if they're not looking better after this rain I'll pull them out and start afresh. Wish they looked like Pete's on Gardening Australia ...

It sounds like you put a lot of "nutrients" in the garden. With leafy or juicy vegetables we generally plant well after the manure etc has been put into the soil. We don't add anything after it has been planted. Perhaps it is affected by this. One year we planted cucumbers with not-so-old chicken manure in the ground and you couldn't eat them. The acid level was so high.

It could be that the old leaves are beginning to rot as will occur if they are not picked...just pull the old leaves out of the plant (at their base) and the new leaves will come up from the centre of the stalk...no problem at all...or you can just leave them there to rot...its natural and does no harm (in a healthy garden)...if you just leave the old leaves on the soil around the plant they will decompose and add nutrients to the soil
The plant will grow all through winter and in spring it will begin to get taller and the leaves will be smaller, more numerous and eventually the pklant will reach about 2m in height and will be covered in seeds which will dry off over summer...the small leaves can still be picked and eaten...try throwing them into a mesclun mix of your own
cheers
Peter

We have previously eaten the outer leaves. The new leaves are the bitter ones. They are definitely stunted looking and not lush and leafy as usual. Do you think I have overdone the nutrients? Or should I concentrate on high nitrogen fertiliser like chicken manure and Charlie Carp?

Maybe the brown tinge is rot starting from the inside...its been a while since you posted this so you may have more of an idea by now
What type of soil do you have and is it high in organic matter or low...is the soil porous and how often are the plants watered
cheers
Peter

I had this problem years ago with silverbeet and so gave up growing it. Very recently I read (on the net I think) the reason for it and the solution was simple - so simple in fact that I promptly forgot it. (I dont grow silverbeet anymore) The answer is out there - it is just a matter of tracking it down. Sorry I cant give you an answer but I hope I have convinced you to keep looking. It was something like adding epsom salt/potassium permanganate to the soil. Someone who has a good understanding of soil science may be able to steer you in the right direction.