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I have some in a pot on the terrace at the moment and it's looking a little yellow. It's growing in rich compost (home made) so I don't think it's a lack of nutrients. Perhaps its the pH?

Equally, I read a blog from the US where the person claimed they left lemon grass plants in the ground outside over winter in zone 9a. That can't be right can it? With sometimes high 20's (°C) in summer and perhaps a few days around -5°C in the winter (perhaps!), am I right in thinking I'm in zone 9a? I can't envisage Lemon grass surviving the winter outside here in Brittany.

You're right - it won't overwinter if it freezes. Not sure what it is resistant down to, but I grow mine outside, and dig it up as soon as frost is eminent. C. citratus is not resistant to frost. Not sure about C. flexuosus, which is East Indian lemongrass, and is the one you have, if you started it from seed. Which do you have?

Maybe what is causing the yellowed leaves is growing it in pure compost. Maybe try mixing it, with some perlite and some peat, or coir.
This will give them some aeration, which may be lacking in the compost.

Here in S. FL, Lemongrass goes crazy if neglected. I had to quit growing it because it was taking over! It likes hot weather and sandy soil in my experience. I also had to fence it in- all my critters loved it too!

Lemon grass will turn yellow and the roots can rot if the soil stays too wet. Compost does not have a lot of nutrients. Although lemon grass is not a heavy feeder, it could probably use a little fertilizer.

Here it's sold as Citronnelle de Madagascar.
I'll feed it a little more and see if that helps, and perhaps more garden soil when I re-pot.