and had been to drop items off for the workmen on site. I understand I was overhanging the footpath but was careful not to enter the road whilst unloading which is evident in the photos, to my mind I have not committed a contravention as I was parked largely on my own property and did not cross the yellow line whatsoever.
At least you're better informed now about yellow lines, their meaning and extent of their restrictions e.g. physically being on the yellow line is not necessary for the contravention to occur.
IMO,
The law is that 'unloading*' is an exemption i.e. the restriction does not apply. But if a motorist claims this exemption then the burden of proof falls to them i.e. they have to prove** that their activities qualify for the exemption.
'had been to drop items off for the workmen on site' is insufficient. You have to establish that the goods were such that it was necessary to use a vehicle for their carriage, not just that it was convenient. In addition, that the vehicle was parked for no longer than necessary to unload* the items.
That the obs period was so short plays to your advantage. Had they observed for 15 minutes then you would be hard pushed to argue that you were parked for no longer than necessary given that the property was adjacent and didn't comprise 20 floors.
So, what were you unloading* and how could you prove this e.g. receipts, statements from the workmen etc?
*- unloading includes delivering which is actually whatyou were doing.
**- the standard of proof for an adjudicator is balance of probabilities. For some authorities it's that it was witnessed by God, and even then they'd probably want further corroboration!