Good Morning Andy,
There is no requirement to have the City & Guilds qualification to PAT test appliances if this is to be done in house. The one day course you refer to will give you enough information so that your team can understand the requirements in terms of classifications of equipment and voltage/fuse ratings etc.
Many clients use the C&G qualifications as a requirement for external partners working as contractors for them. It is a great way to demonstrate competence but as long as you have documented evidence that some appropriate training has taken place and a process of audit/verification of those staff who do the work, I see no reason why you would need to spend extra funds on gaining specific C&G qualifications.
Hope this helps