Regarding composts, I do not know if you are aware or not, Rob - I suspect you might be - but H.W Hyde uses a bracken compost by 'Dalefoot Composts' to grow their show plants in, but I assume they grow in a greenhouse and only one year so would not know how this work long term.
This isn't a specific lily potting mix but if you scroll down a bit in the PDF below you can see what Ian Young uses for his bulbs. I bet it would be very suitable for lilies as well and it certainly is a mix made for your climate. I can't imagine too much water being a problem in such a free draining mix.
http://www.srgc.org.uk/logs/lo...
