This. You'll get a more accurate cut than with an angle grinder. If you do use the grinder, cut a couple of mm on the waste side and sneak up on the line, rather than trying to do an accurate cut.A hacksaw with a decent 24 tooth blade and if you are concerned about the cut a cheap and simple mitre block is your friend and guide. If you go the angle grinder route get the correct disc for aluminium.
thanks for the input all. i ordered this to pickup tonight at screwfix
OX Pro 24tpi Wood/Metal/Plastic Hacksaw 12" (300mm)
upercove Plastic Mitre Box