According to Burncoose:
Half hardy - unheated greenhouse / mild winter - Hardy in coastal and relatively mild parts of the UK except in severe winters and a risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good micro-climate. Likely to be damaged or killed in cold winters. Plant can withstand temperatures down to -5°C (23°F)
I had probably better not... have already got ridiculous quantities of tree seedlings and other impractical things on the go as it is.