These Banksia speciosa seedlings are being grown in UK and the leaves are turning yellow, then brown. What is likely to be causing this and what can I doo to save them?

Also, your seedlings look like they're nutrient deficient/toxicity effected. Are you using a fertiliser in the mix? If you're not, then they're probably starving to death. If you are using a fertiliser, is it a specialised Australian native fertiliser? It must be a specialised fertiliser as Banksia sp. are phosphorous sensitive and will succumb to phosphorous toxicity with standard garden fertilisers.

Have a look at this link from the Association of Societies for Growing Australian Plants on native seed propagation, it might help:

http://asgap.org.au/seed.html

Good luck!

Banksia speciosa, being a western species, is not a particularly hardy Banksia, so I am amazed that being so young they survived last winter outside. (I lived in S London for first 29 years of my life) I would advise keeping your seedlings under glass in winter not only to avoid frost but also too much water.
I am intrigued by the fact that you say that this is the species with which you have had the most success as I have had a failure with B speciosa. I brought a young plant from a nursery last spring and it succombed to too much water in our last rather wet summer in part due to insufficiently drained potting mix. Indeed drainage is all important and for my home grown Banksia seedlings, I use 2 parts peat substitute, 1 part perlite, 1 part pouzzoulane, this last component ensuring good drainage.

As far as curing yellowing is concerned, I endorse the chelated iron/Mg idea, but I’m wondering if the browning might not be due to phosphorous poisoning. I certainly use no fertiliser for seedlings, and generally have good results for Banksias.

How did your 2008 season go ?