+1 for leisureliquids, they have a decent mixing calculator on their website and offer 5% off with POTV5. They have a decent starter kit too and sell measuring cylinders etc to help with mixing. They're also fairly knowledgeable so you can ring them for ideas on which concentrates to get with to give a particular flavour. Start off simple as the Tromain recommends and write down the mixes you do as you go. If you don't like a particular flavour straight away just put it in a cupboard for a while to steep and the flavour might improve with time!