So if I were to be planning on driving a privately owned UK van from Santander down to the Algarve containing a mix of personal hand & power tools to do a bit of work on my property, some 2nd hand & some new furniture for the house, a few new items such as a log burner & electric hob, and some other random bits & bobs, what documentation should I consider having to hand other than - passport, driving licence (pos. IDPs to cover Spain & PT), insurance green card, ferry tickets, receipts for the new goods, fiscal number & every scrap of paper I possess pertaining to the PT property? Oh yeah, GB sticker on the back & beam correctors on the headlights of the van.....
If one of these guia things is needed, how would I go about getting hold of one in advance?

Don´t worry. A van is more likely than a car to be stopped at one of the police controls. In the very unlikley event that you are stopped, once it is obvious that you are transporting private belongings to your new home, that should be the end of things.
The point is that in Portugal commercial vehicles must carry details of their load. I believe that this is aimed at controlling the black and stolen goods markets.
If you examine a purchase receipt here a few have a space for the vhicle registratation number. Even fewer shops ask for this detail so that they can complete the document. Usually it is at places like plumbers and builders merchants where they do lot of their sales to tradesmen.
There is no way that you can get this documentation prior to arriving here.

The only place I have been stopped for this kind of check is in France after crossing the border with Spain. I was driving my Mondeo. They wanted to remove body trim panels, so I assume they were looking for drugs.

I have been stopped just once in Portugal for a routine breath and papers test. However I have driven straight though many such check points since moving here. The paperwork was not 100% correct as I was driving my wife´s car, mine being low on fuel. They accepted my explanation and I was on my way with a military salute and "bom dia, Sr. Alan".

Relax, and enjoy your journey.

Alan

Thanks Alan, I feel a lot better about the whole thing! :-) One can get ones self wound up about officialdom....