I use M&S Heatgen too. I love them because they just look like scoop neck, long sleeve t-shirts - depending on the weather I wear them under a thin cardigan, a thick jumper or just a coat. The colours are great - I've got them in blues, teal, red, purple, black and white (bought over three years).
Are the Royal Family losing their touch?
Cardigans ...longer ones not in fashion this year