Getting multiple versions of what is required for entry ( just for a few days ) to Spain.

Any advice gratefully received.

As a starting point check here, remembering it's unofficial but gives you a good starting point, including links to the official sites:
Where can I travel to? Travel Restrictions by Country | KAYAK

All the facts you need for Spain are here:
Ministerio de Sanidad - Professionals - Travel and COVID-19

You need a vaccination certificate of the vaccine (not booster) from 14 days after the last shot of the vaccine (not booster) and not more than 270 days since the last shot of the vaccine (not booster).
After that you need vaccination certificate showing that you have had a booster shot as well as the full course of the vaccine.

If you don't meet those requirements you need an antigen test taken not more than 24 hours before arrival in Spain/EU (not departure) or a molecular test taken not more than 72 hours before arrival in Spain/EU (not departure).

So in summary:
Vaccinated:
Less than 14 days before arrival, get a test, adhering to the time limits stated above and fill out the form.
OK from day 14 to 270, just fill out the form.
After 270 days you need to get boosted or tested, adhering to the time limits stated above and fill out the form.

Vaccinated and boosted:
Valid from day one of the booster shot. No expiry on the validity at the time of writing (use the link to be sure).

Unvaccinated:
Get tested, adhering to the time limits stated above and fill out the form.

Note: If it's a transit flight it's exactly the same as if you were flying direct as the UK and Canada are both third countries. You need to check the UK for entry requirements specific to your situation, I'm 99% sure there are no restrictions anymore.
If the issue is around needing to be tested in the UK prior to departure then my advice is to avoid the UK for transitting to the EU. Once you're in the EU, you're free to travel around without further tests, however different countries may restrict you from visiting restaurants etc. Check individual countries to be sure.

Avoiding the UK will save you a lot of money on the return leg in airport taxes.

So if we are double vexed and boosted, no need for test ?

We have records of such.

Yes, and complete the passenger locator form.

Thanks

Ok I downloaded the SpTH app onto the phone to complete the name, flight, stay location etc as far as possible. The rest of the form needs completing 48 hours before travel. Hopefully it's the right one.

Yes, it's the correct one. Fairly simple to use, not as good as ArriveCAN but much better than the UK version.

OK great. Done both as far as I can so all ready I think. Thanks very much for your help.