Thanks for the replies. I think I would certainly take the My Heritage estimate with a big pinch of salt. I had a flick through my dna matches and many of them have some weird and wonderful results. Ancestry is a little more accurate, in that it does match with my family tree research.
Like you all say, it is reliant on having a sample population that has very well researched ancestry to match up with dna markers.
It’s a shame that given the majority of my matches on these sites have no family tree that so many people are falling for it! £89 + is a lot to spend on something so inaccurate!