Hi Stewart
I hope you have a copy of the standards. If you haven't, you can download a copy from the City & Guilds website.
Being an assessor, you will be familiar with how the standards are laid out. The principle difference between the level 2 qualification and the level 3 is that it assumed you are in a supervisory roll, hence the unit tiles such as "organise the receipt of goods and materials" etc.
You need to identify which optional units fit best into you daily tasks so that you can provide the evidence more efficiently. Don't go for a unit that "looks interesting" go for the "I do that every day" unit.
Read through what the performance criteria are for your chosen units and then start gathering evidence in the form of documentation and personal accounts of when you have performed to those standards. Bear in mind that evidence for the mandatory units will be covered at the same time if you include how you deal with health and safety issues, interact with your team etc.
Knowledge and understanding can be covered by stating things like "I made sure there was sufficient space to unload the vehicle so that it could be done quickly, efficiently, in a safe manner and was easy to check against the delivery note"
Hope this helps.
Dave