I could really do with some advice please! I work with 1 other person (my line manager) and we have clearly-defined areas of work. I work 2.5 days a week and am usually rushed off my feet (same old story, f/t workload in p/t hours). Today I received an email from him asking me to take on one of his cases. This case has been languishing around since August and he received a chasing email about it. He clearly hasn't even looked at it since August. Now he wants me to take on the case (and presumably take the flack for his lack of attention to it). The case is clearly not my area of work (we are meant to be strict at sticking to our well-defined areas) and I have enough to do as it is. This isn't the first time he's asked me to deal with his mistakes. I've done so in the past but am getting quite fed up of it. My question is, how can I say no to taking this case on without appearing inflexible/unwilling? Thanks all.