I think the manual method may indeed be enough for now.
Most of my clients are other design agencies, so offering pdf is fine for them.
Offering docs as well has so far proven to be enough to cater for the non-design agency clients.
At this time there isn't sufficient legwork involved in cranking out invoices to warrant automation of this kind.
I was possibly being seduced by the idea rather than responding to the need.
Basically just wanted a second opinion, or rather someone to come in and say - don't bother.
Thx for that.