Fixed rate coming to end - credit check to stay with lender??

Hi there,
My fixed rate is due to come to an end with RBS and I am wondering if they will do a credit check if I want to opt for a new fixed rate with them. I have a new loan which may affect how much they lend me, that I didn't have at the start of my current fixed rate. I'd rather not go to their SVR and I've heard that if I stick with same lender they may not do usual credit checks.
Thanks in advance
Sam