i did find ....... a few mites before i put them in.
Think! Doesn't this answer your question before posting this, after being told specifically that the mite is resistant to Bavarol in most areas of the UK?
There will be typically 20% of the total loading as the phoretic stage in the hive at this time of the year.
Seems that your bees may have come from a 'non-resistance' area but that result makes it looks very much like they did not.
Also without any history of the colony we would have no idea if this 'small nuc' was made up a very few days ago, using brood frames from a severely infested colony and 'relatively clean' bees from another source. It happens a lot.
So, yes, you should be concerned.
RAB