I'm told it needs ericaceious compost if I plant it in the garden, but I shall only need a small amount. Would a small bag of peat free compost do the job? I can only find large bags of ericaceous compost,
if you plant it in the garden, you will need enough ericaceous compost to keep it in acid soil for all of its life. What is your soil like? Peat free commercial compost is usually fairly neutral so if your soil isn't acid, rthen it won't do the job. Better to keep the camellia growing in a tub!
When we planted our several camellias 15-30 years ago we knew nothing of special compost requirements, maybe that is why they seem to be thriving. Camellias, given sufficient space in the ground and above it, can grow to a considerable size. Our biggest ones are well over 2m tall and 1.5m in diameter - I have seen larger. They do seem to take a while to get established though, as much as several years, before they really come into their own.