I went out to Bansko Last year 5th March and its was very cold, some days 11 am temp was still -16 to -20. Going again this year 4th March will be taking thermals.
try M & S £10 per Item, or milletts for about the same price.
i didnt take thermals in december and never felt the need for them only had teeshirt and pants on under my ski gear, most people i know who are regular skiers dont wear thermals unless they are going to canada