I've gone back to work part-time and I'm in a sales-related job. My contract says I have a basic salary plus access to a bonus scheme. I have a monthly sales target that I need to meet before any bonus kicks in but I'm struggling to get anywhere close so my employer is now saying because I'm not meeting my target they might have to cut my salary. There's nothing in my contract to say my basic salary is dependant on anything so can they do this?