chrissa - you don't say whether the £120 was the same amount as your husband had been getting. If is was, then it rather looks as if St. Life may have made a mistake & not reduced it as they should have done, unless there is something in the T & C which allowed it to continue at the full rate for a fixed period after his death.
If the £120 was a reduced amount (normally 50% of the full rate) then it is difficult to see why it should now be halving again & you certainly need to have a proper explanation of it.
Also, they should send you a copy of the letter they were referring to when you phoned, or - if they didn't send one - send one now with a proper explanation.