Dear X
I am writing to you in relation to my current workload and the request that I undertake (X) marking. I have been asked to complete this work by (date).
I am currently working at full capacity. Undertaking this additional marking would put me in excess of my contractual hours. I will therefore be unable to undertake this additional work.
Should you wish for me to complete this marking, please confirm which work should be prioritised within my contractual hours and the work that I do not need to complete.
I look forward to hearing from you.
Kind regards,