Yep Paul, you're totally right
It makes me more confident in the fact that my engine could actually be OK. But I want to be 100% sure.
The block testers are really expensive here (£70!), but I could manage to have one lent by the local garage, I just have to pay for the fluid. So if everything goes fine, I'll do the test tomorrow, and I'll know for sure if the head and/or head gasket have problems.
The spare engine I found looks to be in a good condition if I refer to the pictures I received today, so the alternative is ready anyway!
Thanks again for the help.