For an all products approach you can look at editing the Currency Formatting (and just adding text like inc VAT directly):
Now if you want to have variable display like "per dozen", "per m", etc you'd need to look into some theme edits. Now the big question to start with is how are you setting that value on the product in first place?
Would the custom text be different per variant, or the same for all variants on a product?
Got an example link to a product that will need this? Sometimes seeing a real product and working link can give a bit more insight into the theme and possible approaches.
Thanks for your quick reply. It would purely be a all products approach so I guess your first recommendation would be the best. Looking into those settings it is currently showing the following:
HTML with currency
HTML without currency
Email with currency
Email without currency
How would I apply the code to show 'Inc Vat'