Well I carried out some tests with the magnets today, it would appear the external magnets may be as effective, if not more so than the Votex.
I tested the Votex against 2,3 &4 external mounted magnets, from observation the sump plug with the external 20mmx5mm magnets had larger field effect and more load (iron filings) carrying capacity.
Measuring actual microteslas, background reading was 261uT, the Votex measured 276uT, the sump plug with 2magnets measured 288uT, three magnets 312uT, four magnets 343uT.
I then laid a trail of fine metal filings in a thin line and observed how far the magnetic field was having an effect, with 4 magnets the effect could be seen for approx 50mm, with the votex it was approx half that.
Then I placed some 30w engine oil in a plastic container and sprinkled iron filings into the oil, while observing the Votex and the magnets at opposite ends, due to the physical size of the whole sump plug being magnetised, it captured much more metal. Having said that the attractive force of the Votex was extremely good when filings went past in close proximity.
I have attached some photo's to give an idea of the tests, there is one more test I would like to do, which is use a sump plug with a thinner head more like the Hyundai original, I have one ordered but can't pick it up until I have a negative RAT test.