Sick chard, any ideas?

It could be a virus - or maybe magnesium deficiency - see if Epsom salts help.

Looks like a sort of rust? Does it affect the taste? I would cut the plant right back. It will soon grow and you can see if the new growth is affected.
This weather is to blame for a lot of problems!!!

What's the new growth like?

The affected leaf does look like an old one and there do seem to be the sort of lesions which fungal attacks can produce. Maybe it's just had the worst of the wet weather and with any luck if you take them off the new ones will be more lucky. 

I think it is perhaps old age 

Don't write yourself off just yet.

I'm going to cut them off and see how they grow back.  This has been a horrible season to predict.  No slug damage at all anywhere, but a plague of whitefly, greenfly and blackfly on a scale of which I havn't seen before.