I read that the building regulations stipulate that there must be 400mm from the door or swing of the door to the bottom of the stairs, does this apply to pocket doors as well?

Check out ADK https://www.gov.uk/government/collections/approved-documents it looks like it only applies to swing doors but applying it to a slide door would only give you 400mm before you have to step up I think I would give it more if possible, check it out with Building Control. 

The 400mm rule is a relaxation and applies only to a private stair. Private or otherwise, there is nothing in the technical booklet that would suggest consideration needs to be afforded for pocket doors on to stairs. I am in NI, still part of this completely disunited United Kingdom!