I am looking to obtain a copy of the SP10/91, but can't seem to find it on HMRC website.
Does anyone know where I can get a copy?
You can get it on HMRC's website. :) See http://www.hmrc.gov.uk/agents/sop.pdf and use the find tool in your pdf reader to find SP10/91.
http://www.hmrc.gov.uk/agents/sop.pdf will give you ALL the statements of practice.
Don't attempt to read the whole document - 355 pages in all. The last person that tried it is still in a coma. You just need to go to page 111.