After the 10 months you will have a full proper +1 to your ncb that other insurers will accept
No, you will not.
I tried this with Norwich Union, and when I went to Endsleigh they only accepted the NCB as I had been with NU for a year anyway. Had it been 10 months only NU would have accepted it, which was a lot higher on my premium than Endsleigh were.
InvG
With EUI (admiral, elephant, bell) the 10 months is valid as 1 year with other insurance companies
With EUI (admiral, elephant, bell) the 10 months is valid as 1 year with other insurance companies