I think i've reached the end of the road with this search. My main 12 word phrase does not seem to work in either mycelium, electrum, bitaddress.org. Not tried Multibit; but I'm sure it won't work there either. I'm gutted; and also annoyed because I know it should work.
You probably misspelled your 12 word phrase or something like that, which is why it your seed-phrase doesn't work in wallets. In any case, some mistake was made, otherwise you would have already gained access to your funds.
The thing I found strange was that my other 12 word phrase that worked for Electrum and revealed historical transactions also worked in Mycelium.
It seems to me that there is nothing strange in this, because 12 words doesn't contain errors, which means that it works for Electrum or Mycelium.
I'm going to park this problem for a while till i get fresh ideas; but thank you to everyone that helped esp. nc50lc and ETFbitcoin.
Thank you.
Good decision. Perhaps a break will do you good and you will notice those details that you might have missed.