The MSD pages in the HSE website refer to their publication 5 Steps to Risk Assessment. However, you may need to consider engaging a competent ergonomist to help you complete the assessment.
The causes of MSDs can be wide and diverse, but many of them are related to ergonomic aspects. The ergonomic assessment form (see occupational hygiene and ergonomics) covers many of the common issues and can be used to help identify the relevant hazards in an MSD assessment.
You may also find that a review of your risk assessments on manual handling and display screen equipment (DSE) (see risk assessments) can be useful inputs into your MSD assessment.