Recently went through similar with two associated businesses. The sticking point mainly is the number of redundancies involved. If its under 20 then its a lot simpler process (the consultation process is reduced, and you don’t have to notify the RPO prior to commencement).
Having said that the RPO office is probably the best source of information, and will on occasion provide a consultation service. The Government Gateway and the Businesslink website also lay out the complete process from start to finish.
We also used Mentor (who are part of the RBS group) who were exceptionally good, they may be able to offer assistance as a one off (we have them as underwriters and HR advisors on a permanent basis)
Going forward, I would recommend retaining a similar service, its costs me just under £100 month for having under 20 employees, and provides full HR consultation, and litigation cover.
Worthwhile contacting your local chamber of commerce as well, as i know ours offers a similar service for members as the RBS.