Thanks for all your input, here's an update.
I had a good look on the good ol interwebs after a good heater. I found cheap 1500w ones from china being sold all over amazon and other websites, or there were a few other options with Russian name and text on them but they are easily found directly on Chinese sites as well.
I didn't feel like putting some no-brand china unit in the car as the consequences of a malfunction would be severe.
Found this one sold by a small Swedish company, surely imported somewhere from china as everything is, but at least I will have protection through Swedish consumer Laws if something were to happen.
It is 1.1kw with built in pump, cost equivalent to £130 with shipping, feels like good build quality and it has a serious electrical connector made for pre heaters and coupe heaters, and I already had a heater inside the car using that system.
I went ahead and mounted it on the hose between the expansion tank and connector on the hose between engine and heater matrix with the thought that the water would have to flow through either heater matrix or engine.
First placement didn't work as a problem with air inside it occured, fortunately it survived as I realized this risk and and filled the heater before mounting it.
Moved it a bit on the same hose to a better location where the air would escape easier, and this time it worked, but...
It isn't clear in the flow chart, but the water doesn't have to take its way through the engine or heater matrix, it just "turned" the wrong way and returned to the tank through the water pump and return hose, so nothing got heated apart from the water in those two hoses and the tank.
I got mad and yesterday I finally relocated it to one of the hoses between the engine and the heater matrix.
Made room in the garage, put the front end on stands, got under it, removed the plastic underneath the engine, squeezed my hands in to the tiny space where it could fit and got to work.
Many swear words and some coolant in the eye later, I got it in place.
Tested it this morning, it ran for 40-45 minutes before reaching it's set temp where it stops until the temp drops.
I got in to the car and to my surprise the coolant temp read 75⁰c.
It was +4⁰c outside (car says +6⁰c in picture but that's due to the heater, and during the night the temp were down to 1⁰c), so not really that cold but still.
It appears to circulate the water through all of the engine and heater matrix as the temp didn't drop at all when I started the car.
The manual says it should stop at 55⁰c and start again below 40⁰c, but it seems like it goes higher as the car indicated higher temp.
Great success and so worth all of the trubble. A DEFA heater wouldn't come near these results.